Personally I've just given up completely on arguing with the book-based belief that Leviticus 20:13 or whatever. That belief isn't adopted rationally so it can't be dispelled rationally. I don't like it. I would prefer for it to go away. It might go away on its own time, it might not, but there's nothing to be gained by railing against it.
In the other thread I asked my RMHA comrades to lay off at the point where the only vestige of heterosexism is private disapproval of the sort I've called irrational. I will continue to discourage that badgering. It's not the way to change minds.
I do think the analogy with rasism is apt. We all agree that there is nothing about race that justifies discrimination. If the analogy bothers you, if you feel outraged at being compared to a rasist, well, you have a good grasp on how fundamentally wrong heterosexism is to us. I don't expect you to agree, but it's good that you understand.
And I'm hopeful. We've made enormous progress. The reality of homosexuality - we're just like you - is becoming more visible. People are noticing that children aren't screwed up by being raised by gay parents. Same-sex marriages haven't damaged opposite-sex marriages. When society doesn't marginalize them, queers live lives that are identical in every way to straights'. The goal is for it to become a nonissue.
I guess that depends on what you mean by "rationally." It's not something most people (perhaps anyone) would come to through the use of pure reason, but then I'm not sure we could honestly say the same of any ethical principles (Kant aside). I think it's a rational extension of beliefs that are reasonable to hold, which qualifies it as rational under what I think is a reasonable definition.

It can be argued against, it just can't be argued against
very easily, because it's something not fundamentally disprovable by empirical means. But the same could be said of many ethical ideas -- this is just the latest unpopular one.
See, I really don't think the racism comparison is a very good one. To explain why, though, takes a bit of doing, so bear with me.
Imagine, if you will, a man named Mr. Jones. He is, for all intents and purposes, omnisexual -- men, women, transgender, furries, whatever, they all potentially work for him. But Mr. Jones is not, and never will be, attracted to black people. Pretty, handsome, ugly, grotesque, young, old, kinky, vanilla, whatever -- if they're black, then they just won't work for Mr. Jones. He's not a racist per se -- he doesn't think blacks shouldn't be allowed to vote, and they thinks they should be treated the same by the government, and so on, but he's just not sexually attracted to them in the slightest, and he's perfectly willing to state loudly that he thinks they're all sexually unattractive if asked. (Although he's also willing to admit that this is a personal opinion.)
Now, I don't think any of us would want to throw Mr. Jones in prison. Most of us would probably not even say he's a bad person -- he can't exactly help who he's attracted to, although he could probably let up a bit on the public declarations of unattractiveness. But there does seem to be something....unfortunate, at the very least in his inability to find any black person sexually attractive. It's not a moral failing, perhaps, but it does seem to be something that we would normally prefer not happen.
If you're not so sure, consider instead if this was not his 'natural' state, but he had some sort of special treatment that rendered him unable to see black people as sexually attractive. (And perhaps he went further and made himself unable to see Asians, Hispanics, and all other non-white people as unattractive.) There seems to be something very...ugly in that, even though it's his own sense of attraction, and nothing else.
But what's the difference between this and heterosexuality or homosexuality? I've met many people, particularly straight males, who are willing to proclaim quite publicly that they're not attracted to men, and never will be. (I'm one of them. And no, I'm not secretly gay.

) I've met gay people who are turned off by the opposite sex, too -- lesbians who think penises are freaky and men unattractive, gay men who think vaginas are disgusting and women sexually unattractive, etc. And no one seems to really have a problem with this.
But if race and gender/sex are analogous, then why not? We seem to prefer that people have multiracial preferences, but don't seem at all bothered if they do not, or refuse to have multigender sexual preferences. Now, it could simply be that we have more interracial relationships than bisexual examples, so we're more normalized in that respect. But I don't think that's it. I don't see, even in the gay rights community, a secret belief that bisexuality is superior. It seems much more reasonable to me to think that biological sex, and gender, as a much more fundamental aspect of human relationships and human society than race; some form of gender is necessary, some form of race is not. But if one is more fundamental or important than another, then I don't see why ethical beliefs about one must be the same as ethical beliefs about the other, as they have important differences.
So that's the long answer.

(This argument is not wholly original to me, either -- I've seen it made before in a slightly different form, although I cannot find out where.) And no, I'm not outraged at being compared to a rasist, as I don't think it's a very good comparison. I'm slightly saddened, but not outraged.