Yes, you are thinking of a different way than I am. This is what I thought you were saying, but it's so completely different from any comparison I've ever made that I thought I must be misunderstanding you.
I read your analogy as an attempt to establish that race and sex as factors in sexual attraction are not comparable. Therefore it's reasonable for us to have expectations for sex's role in sexual attraction, but not race's role in sexual attraction.
I think your reasoning betrays itself, though. In establishing difference, you asserted that we find sexual attraction as a function of race less acceptable than sexual attraction as a function of sex. It hinges on variations of sex motivating attraction. How can you reconcile that with disapproval of certain sex-motivated attraction?
Now I'm lost.

I really don't understand the question.
So yeah, we weren't on the same page, and I think it was my fault. I don't actually agree that preference for various physical features is in any way objectionable, but that's not really relevant.
Not objectionable, but somehow unfortunate if they're extremely limiting. It's not just general physical features -- like liking brunettes or big boobs or whatever -- but a categorical and arbitrary delineation that strictly separates humanity. White versus non-white, for instance. I don't think it's particularly unfortunate if you have a thing for blonds, but I do think it's kind of unfortunate if you're physically incapable of being attracted to anyone who isn't of European descent. Do you disagree?
What I assume you mean by "religious beliefs on the subject" includes that there is something bad about homosexuality. Only heterosexual couples are able to be good, whole, natural, healthy, moral... homosexual couples are second-class. For better or worse, that is heterosexism (homophobia). When these things are thought of race - that there are superior and inferior types - we have rasism.
So what's the difference? We agree that race doesn't justify discrimination. But to you, heterosexism is justifiable, because the book says don't be gay, because being gay is inferior to being heterosexual. That belief is harmful. It is equivalent to believing that being brown is inferior to being white: you consequently discriminate for no good reason. You believe you have a justification for it, but it's no more truthful than anything rasists have ever used to justify their beliefs.
Well, for starters, I don't think your overall characterization is entirely fair -- if we're going to argue the Christian position (And the Abrahamic one more generally) no one can really be entirely good, whole, natural, healthy, or moral. Some are less bad than others, but none are truly good. (It doesn't resolve the problem you're proposing, but it's an important point to note.)
Second, remember we're not talking about discrimination per se, just belief about the sinfulness of a particular thing.
Third, you're using a circular argument here. You're starting with the assumption that your opponent's belief (Gay sex is sinful) is wrong, and then building outward from that a comparison to racism. Your argument could be succinctly if simplistically stated as "since it's not wrong to have gay sex, it's wrong to think that having gay sex is wrong." While logical, it's not a particularly helpful argument to make.
Finally, there's nothing in my Bible that says "being gay is inferior to being heterosexual." You're making this a matter of identities, when I'm talking about actions. Being black isn't really an action; having gay sex is. It'd work better if you limited it to, say, opposing interracial sex on racial grounds alone. Working back from there, you could reasonably say that the basis for that moral judgment is based on race -- specifically, an idea (and a twisted one) about the values or compatibilities of different races. Calling this racism makes sense for that reason.
But what's behind the idea that having gay sex is wrong? It's not actually a statement about orientation -- I've never heard a Christian argue that it's only wrong to have gay sex if you're gay, 'cause that'd be pretty weird. (I've heard a few liberal Christians, like pasi, say it's only wrong if you're straight, but that's contrary to the general orthodoxy, and silly in my opinion.) Rather, what's behind it is fundamentally ideas about biological
sex, and the related notion of gender. Even if we accept that these ideas are wrong, it'd be more accurate to label them as some sort of sexism or genderism rather than heterosexism. It's about sex, not orientation. (So derogatorily label us accordingly!

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