Catholics assert that obedience to Gods will is good firstly simply because God is God and seeking God is a good end in and of itself, and secondly by virtue that God created the universe. Ergo its his game so to speak and so he is arbiter that determines its rules.
There are two distinct claims here, which you're running together.
First, there is the claim that (1) obeying God is good.
Second, there is the claim that (2)
only obeying God is good, or, if you prefer, that obedience to God is the sole good-making characteristic of any act.
What you've said in the above quotation supports (1). And indeed (1) seems perfectly reasonable. If God exists and is the perfectly good creator, then yes, obeying him would be right. Not only that, but it would be right to obey him in matters that would otherwise be morally indifferent; e.g. if God commands everyone to drive on the left instead of the right, it would be right to do so simply because he has commanded it, even though there's no intrinsic moral rightness or wrongness about driving on one side of the road rather than the other.
But just because (1) is true (and, if God exists, it is true), that doesn't get you to (2). The reason for this is that (1) is plausible because it rests on a general moral principle, which is that the wishes of someone to whom you owe a great deal are to be respected. E.g. children should, other thing being equal, obey their parents; I should, other things being equal, obey the law. You said this yourself when you said that the universe is God's game and he gets to lay down the rules; if he is our creator then we owe him our obedience.
But if you're trying to defend not claim (1) but claim (2), you can't do this, because claim (2) states that all moral principles derive from God. But this would include the principle that it is right to obey the commands of someone to whom you owe a great deal (or in this case, your existence); and it's wrong to disobey them. Where does that principle come from? It can't be laid down by God, because then the argument would be circular: you'd be saying that all moral principles derive from God because of this other moral principle that derives from God. But if it's not laid down by God, then there is at least one moral principle that doesn't derive from God. And indeed some defenders of divine command theory have tried to argue this, but to my mind at least it doesn't make sense; if you're going to accept that there are any moral principles that are independent of God then I don't see why the rest of them can't be too.
However on the main point you make, would it not be logically absurd to say some law in the universe, or some matter, exists independently of God as creator? I mean, to say that that thing (be it the laws of mathematics or some portion of the material stuff in the universe) was not created by God, which would imply the universe including its matter and its laws was not created by God (which is against the definition of God as creator). . You can't say that mathematical laws (rather than pure chaos, or some strange reality incomprehensible to our limited minds bound up in the laws we operate within) exist independently of God unless you deny God exists, since if the Universe is created by God than by definition he created the mathematical laws that underpin it. Indeed since God by definition is the first principle and eternal (timeless), than if God exists than you can't say anything is independent of Him (either as part of his essential nature, or by his decree in the act of creation) since everything would be contingent on God.
Some philosophers and theologians in the past have thought this, most notably Descartes, who got into a bit of trouble for it. Peter Damian is often cited as defending a view like this too, though I think he didn't really do so. However, the vast majority of classical theists have rejected it, and held that necessary truths are true independent of God's will. That is, God does not have the power to determine what is (logically) possible; he has only the power to actualise anything that
is (logically) possible. So, for example, the fact that 2+2=4 is necessarily true quite apart from anything God wills, and God could not change that fact if he wanted to (not that he would want to). Similarly, God cannot create a square circle, because such a thing is a contradiction in terms, and the fact that it's a contradiction in terms has nothing to do with God's will.
This is the standard Catholic view. Thus, in the
ST I.25.3:
Thomas Aquinas said:
All confess that God is omnipotent; but it seems difficult to explain in what His omnipotence precisely consists: for there may be doubt as to the precise meaning of the word 'all' when we say that God can do all things. If, however, we consider the matter aright, since power is said in reference to possible things, this phrase, "God can do all things," is rightly understood to mean that God can do all things that are possible; and for this reason He is said to be omnipotent. Now according to the Philosopher (Metaph. v, 17), a thing is said to be possible in two ways.
First in relation to some power, thus whatever is subject to human power is said to be possible to man.
Secondly absolutely, on account of the relation in which the very terms stand to each other. Now God cannot be said to be omnipotent through being able to do all things that are possible to created nature; for the divine power extends farther than that. If, however, we were to say that God is omnipotent because He can do all things that are possible to His power, there would be a vicious circle in explaining the nature of His power. For this would be saying nothing else but that God is omnipotent, because He can do all that He is able to do.
It remains therefore, that God is called omnipotent because He can do all things that are possible absolutely; which is the second way of saying a thing is possible. For a thing is said to be possible or impossible absolutely, according to the relation in which the very terms stand to one another, possible if the predicate is not incompatible with the subject, as that Socrates sits; and absolutely impossible when the predicate is altogether incompatible with the subject, as, for instance, that a man is a donkey.
It must, however, be remembered that since every agent produces an effect like itself, to each active power there corresponds a thing possible as its proper object according to the nature of that act on which its active power is founded; for instance, the power of giving warmth is related as to its proper object to the being capable of being warmed. The divine existence, however, upon which the nature of power in God is founded, is infinite, and is not limited to any genus of being; but possesses within itself the perfection of all being. Whence, whatsoever has or can have the nature of being, is numbered among the absolutely possible things, in respect of which God is called omnipotent. Now nothing is opposed to the idea of being except non-being. Therefore, that which implies being and non-being at the same time is repugnant to the idea of an absolutely possible thing, within the scope of the divine omnipotence. For such cannot come under the divine omnipotence, not because of any defect in the power of God, but because it has not the nature of a feasible or possible thing. Therefore, everything that does not imply a contradiction in terms, is numbered amongst those possible things, in respect of which God is called omnipotent: whereas whatever implies contradiction does not come within the scope of divine omnipotence, because it cannot have the aspect of possibility. Hence it is better to say that such things cannot be done, than that God cannot do them. Nor is this contrary to the word of the angel, saying: "No word shall be impossible with God." For whatever implies a contradiction cannot be a word, because no intellect can possibly conceive such a thing.
There is of course nothing in this to contradict the idea that God is the creator of everything, since possibilities aren't things, and neither are necessary truths. God is the creator of all actual things other than himself, including the whole universe and everything in it; it doesn't follow from this that he's the creator of the logical laws (as opposed to the physical laws) by which all possible universes run, because those aren't really things, they're just necessary facts.
Your point on suffering just reiterates what I said about a moral system divorced from God being entirely based in human opinion. We COULD rank actions according to how much suffering they cause, and morally rank agents according to similar criteria, however there is no universal objectivity to that moral assertion and its fundamental reduction is pure and simple human opinion. Thus while it is fair to say that it is a moral code, it is not correct to say that it is an objective and universal morality that is extrinsic to human beings. It would be a fabrication of the human intellect and one could not argue reasonably for a distinction in validity between that moral code and any other.
But you're not giving any reason for this view. You say that if we were to rank actions according to the suffering they cause, there would be "no universal objectivity to that moral assertion". Evidently you think not, but what's the argument for this view?
You must distinguish between two different claims here:
(1) It is possible (i.e. it could have been true) that "it is wrong to cause suffering" is an objective moral fact that is necessarily true independently of the will of God.
(2) It is true that "it is wrong to cause suffering" is an objective moral fact that is necessarily true independently of the will of God.
Now clearly you reject (2) because it is inconsistent with your God-based metaethics. But I'm not asserting (2). I'm only asserting (1). This is because you're claiming that your God-based metaethics is not only
true, but the
only possible system under which moral claims have objective truth. This is an extremely strong assertion and one that seems, on the face of it, false, given that we can easily imagine alternative systems in which moral claims have objective truth. These systems don't need to be true to be counter-examples to your claim, they only need to be possible.
So imagine, if you will, that moral truths are true in the same way that Aquinas says necessary truths are true, i.e. because their denial would involve an inherent contradiction, not because of the will of God. You will surely agree that (1) this is a comprehensible claim and describes a possible way that things could have been, at least as far as we can tell, even if you think it's not how things actually are. You must also agree that (2) if this situation were to obtain, moral truths would be objectively true and not a mere matter of human opinion. But if you agree to these two things, you must agree that (3) it's wrong to state that divine command theory is the only way to secure the objectivity of moral truths and that all alternatives are mere relativism. If you want to continue to deny (3) then you must say which of (1) and (2) you reject, and why. Because both (1) and (2) seem to be true.
With regards to morality being dependant on God however, it is of course arbitrary in the sense God chose a particular order rather than another (as is true any other aspect of creation). Morality however could be said to be universal, and to be extrinsic to human whim precisely because God is eternal and the creator. As I said its his game and so his rules apply, and an objective morality is such precisely because it has been deemed thus by the creator.
As I said above, the argument that "it's his game and so his rules apply" appeals to a moral principle (i.e. whoever's game it is, gets to make the rules, such that disobeying them is immoral), which is fatal to the argument. Does that moral principle depend on God's will or not? If yes, morality is circular; if no, morality isn't down to God after all.
If alternate universes with opposing moral codes do not exist, that does not make the "necessary truth' independent of God. Indeed what would make that truth true would be precisely that a given moral truth proceeds from the Eternal Gods uncreated nature, being instituted by God into His creation (both generally and in the limited sense of humanity proper) as a reflection of Himself. Ergo that moral law would be objective in origin and be necessary precisely because it is essential to the divine being whom ordered the universe. It wouldn't be extraneous to or independent of God, but proceed necessarily from Him.
I find it very hard to understand what this would involve. I can understand the idea that charity is good because God commands it, on the understanding that he could have commanded something different. I think it's a mistaken idea, but I understand what's being said there. But what does it mean to say that the goodness of charity just emanates from God necessarily, and not as a consequence of his will? What makes it good? The fact that it reflects (in some way) the way that God is, or the way that God behaves? Well, perhaps that can make sense. But it seems to be inadequate to explain
morality. Perhaps charity is good, in the sense of admirable or beautiful or desirable, because it reflects God's nature. But I could say, without any apparent inconsistency, that I think it's good in that sense, whilst denying that I'm under any moral obligation to perform it. Just as I can say that Michelangelo's David is a good sculpture without feeling under any moral obligation to carve something similar myself. Morality is about obligation and imperative - if an act is moral, it's not just something to admire, it's something that
I ought to do. And if an act is immoral, it's not just something to abhor, it's something that
I ought not to do. God is a perfectly admirable being, and we may say that what comes from God is as perfectly admirable as anything can be that isn't God; but you can't get moral imperatives from that.
(You
can get moral imperatives from divine
commands, for the reasons I gave above; so in that sense, divine command theory is superior to this sort of divine essence theory, at least as an explanation of morality. The problem with divine command theory is that, as I've said, you can't base the
whole moral system on it; you can only say that some things are right or wrong because of God's commands, not that God's commands are what determine all rightness and wrongness.)
I'll accept that if this is your view, it's not relativist. But I think it buys that non-relativism at too high a price, namely the price of not really being an explanation of morality at all. It's more like an explanation of beauty. (And that's hardly surprising, given that it basically
is the Neoplatonic explanation of beauty, recast into moral language.)
On your hypothetical alternate universes. If alternative universes with opposing moral codes do exist, and what is evil in one is good in another are different (presuming that men in both are otherwise identical) the laws in each one would still both proceed from the one thing that is non-contingent (God) and eternal, which is the creator of both universes. They would thus be objective in the sense that they are universally valid as implementations of the divine will, and objective in the very basest since that morality would be reduced to "what God decrees as creator is law" and thus universally valid. A non-theistic conception couldn't claim this precisely because there would be no reasonable basis for any one conception to have higher validity than another, since all conceptions would be simply products of human beings.
Sure, they'd be objectively true for the people in that universe, but obviously not necessarily true; just as it's objectively true for us that light travels at 299,792,458 metres per second, but presumably there are possible universes in which it doesn't. But I still say that there's a worrying modal relativism about this idea, because it would accept that morality could have been different, which means there's nothing inherently good about charity or inherently bad about murder.
) the very reason sex exists is due to the biological imperative of procreation, with all other aspects of sex (such as pleasure) being entirely contingent, and indeed oriented to serve this end (pleasure existing so that it is enjoyable, thus supporting procreation by making sex a desirable activity). Your assertion that reproduction is a byproduct (in the sense of ends) of sex is simply absurd from a biological point, and from any rational perspective, it is quite clear procreation is the direct objective. Now this applies to morality in the Christian perpective, since Christianity upholds that God created the universe and established within it an order. If the ultimate end of the Christian is to obtain the beatific vision through following Gods law, than acting in harmony with the order of nature obtains a moral character, since the entire definition of morality hangs upon becoming closer to the divine. Ergo what is in accord with the order of God's creation and is in harmony with his will and divine plan is moral, and what is not is immoral. Since God created the universe and its laws, and since sex is biologically in the laws of nature oriented towards procreation, to the Christian it is morally right to act in harmony with the inherent order of nature, and with the natural end of the sexual act as is imperative biologically, within the context of the end of the human being himself to achieve the beatific vision. 
