But why do you believe this? How is it that there's actually an answer to what is morally right, or is it just your personal preference?
It is self-evident in many cases and has absolutely nothing to do with opinion, mine or otherwise. Unless you count the definitions of words as being subjective. They can be used subjectively, but can also have objectively defined constraints.
Let's say we weren't talking about morality, but we were talking about chess. Is a move that we make right or wrong? Well, the answer is complicated, and there isn't always one correct answer, but sometimes, there is an objectively correct answer. If you make a move which leaves you vulnerable to a checkmate in one, and you had other, better options, then you made the wrong move. If you make a move which is checkmate, you've made the correct move. If you make a move which creates a forced checkmate in the least number of moves, it is the correct move. In some positions, the outcomes of every move can be fully and completely calculated out, leading to objectively correct or incorrect moves, some to varying degrees of correctness or incorrectness. These could be quantified mathematically- forced mate in one, forced mate in two, a forced draw, or a forced mate against you in X number of moves, as ranked correctness. In any position where the possibilities of every outcome hasn't been fully analyzed (yet), and as is pertinent to real life where the outcomes of our actions cannot always be fully analyzed, there are still some objectively correct or incorrect choices that we can make.
I can choose to try to forcibly rape someone, for example. And I could lay out a very objective and rational case for why that is self-evidently immoral.
Taking rape as an example for the purposes of this discussion, because it is something very self-evident and quite uncomplicated. (Morality is not binary or absolute, however, and some moral questions are difficult to guess.)
If we are to objectively define "incorrect" moral choices in this discussion, we could define them quite similarly to the above example regarding chess. In chess, what is correct is what leads to a win, or a draw if the alternative is a loss. What is incorrect, in the field of morality, depends on what "moral" means. If moral means whatever I want it to mean, then it is simply opinion and everyone has opinions and they sometimes are in conflict.
But if you define morality in objective terms, such as objective "harm" or objective "necessity", you start to get somewhere.
For example, in a moral thought experiment, you are asked to give your response to one of the examples of the
Trolley Problem, there are choices presented to you. Doing nothing is a choice, and your choices are limited to what is actually a real option to you (wishing or magic is not allowed, you're bound by the laws of physics and the constraints of the real world, as you would be in real life moral decisions).
Under these defined parameters, you are placed in a situation where you are forced to make a choice that you cannot avoid, and inaction is a logical choice.
This situation can be discussed quite objectively. "Necessity" here can be defined as the fact that you are in this situation and have no choice but to make a choice. Necessity can also be defined as the limited options presented to you. To avoid outcome A, necessity dictates you must choose a different option, and the options are limited.
Those terms can be defined, rigorously, moreso than I have done here for illustrative purposes. "Harm" can be defined as well, as the likelihood or near-certainty of death is harm by an objective and logical definition.
Using these terms and standards, we could observe that all outcomes lead to harm or near certainty of harm, and necessity dictates that one outcome will be the result. Under those constraints, one could argue the action taken which results in the least harm, is objectively the least immoral choice.
Rape, on the other hand, is not something that lends itself to a
necessity. While one could construct absurd thought experiments (less plausible than the trolley one) where
somehow, a rape was the least immoral choice, in the real world and in everyday situations, rape is a willful choice that is never the least immoral one. Inaction would be less immoral in basically all situations, by an objective definition of "immoral", meaning harmful. And one does not realistically consciously and without being mentally impaired put themselves into situations where inaction is no longer an option, as pertains to rape.
Therefore, it is quite easy and self-evident to show that rape is by its intrinsic nature always an immoral choice, and realistically, only in carefully constructed and definitely absurd thought experiments could it ever be considered the least bad choice. As such, rape is for all intents and purposes objectively immoral.
At no point is personal opinion entering into this. It all flows rationally, logically, and consistently from purely objective definitions. The only conceit is that we're describing something using terminology that suffers from the imprecision of language itself, which is why we have to be rigorous when defining terms. Otherwise, the concepts stand on their own merits.
Some situations or positions are more unclear, but that is the easiest example I can think of to demonstrate why an objective morality or at the very least, immorality, exists, in a manner we can define with language.
What remains is to quibble over whether or not rape is always harmful, or to quibble over the definition of harm. But this is merely semantics, akin to arguing that two doesn't actually mean two, because either one of the two could be split into fractions of a whole, and therefore are not always objectively two items forever. But that is semantics, 2 = 2 is an objective mathematical truth, supported by the self-evident nature of reality.
Rape is supportably and self-evidently immoral, by definition. In essentially all circumstances. I only need to prove it by defining it, the evidence is itself. It's not even a difficult question. The only "opinions" entered into the discussion would be that words can be defined and used with logical self-consistency and objective or scientific rigour in the field of moral philosophy. Which is necessary to construct any theory of objective morality anyway.
I also take issue with one specific thing: the claim that if no objective morality exists, then morality as a whole doesn't exist. It can absolutely still exist, but in a diminished form: it's just a set of preferences built into a human brain by evolutionary processes.
I can accept that there are behavioral preferences in a human brain, but given that my own opinion on what is moral has changed over the years, I do not believe that those preferences are absolute and solely a result of evolutionary process. If I can be persuaded to change my position, then evolution has not forced me to take a position.
Some moralities work better than others at keeping human society functioning well enough to provide a survival advantage, so naturally we ended up with moral preferences that mirror that. It's still not clear that this proves slavery wrong, but there are good game-theoretic reasons to believe that a society that doesn't punish murderers while valuing sharing of food is not going to make it for long against a society that has the opposite preference.
Some moral questions may be difficult to answer or are unanswerable in our lifetime. Morality is not a solved game, nor is life, nor (at the present) is chess. Even given our incomplete understanding, knowledge is still possible, and some things are knowable.
Suppose life is like a chess position which has not been fully calculated, all the outcomes are not known yet. But, one can still objectively make a blunder that leads to a forced loss.
Morality is somewhat analogous, in that while there is much we still do not know about it, and it is beyond our capability to fully and exhaustively analyze every single possible moral predicament, some things are still objectively knowable.
Rape is immoral, and it is a (fairly easily) objectively knowable and definable moral stance to have, and factors such as popularity or tradition or my own personal opinions or the culture I am a part of have nothing to do with it. One simply accepts language as being definable, and the rest follows.
If one defines morality as "whatever the mob decides" then, sure, morality is subjective. But then it isn't
morality. Then you're taking a distinct other concept (popularity) and
amending its definition to suit your purposes, contrary to what an objective measure would be.
Difficult to define, difficult to argue, and easy to dismiss, certainly. But if people are actually concerned with "what
is moral?" as opposed to "what
I want?" then they have to accept the premise of objective terminology.