David Velleman at Left2Right raises some worthwhile questions about asexuality and marriage. His argument is that some people don't have sexual attraction to people of the opposite sex not because they have it to people of the same sex but because they don't have it at all. They're asexual. Yet we allow these people to marry, and we allow them to marry each other. We just don't let two asexual men or two asexual women marry each other. On this one I don't agree with him fully, but it's the kind of question worth asking.
I agree that those who think homosexuality is unnatural should think asexuality is unnatural. I think even those who don't think homosexuality is unnatural should think asexuality is unnatural. On the other hand, its being unnatural might not be of the same sort. What I'm worried about is the connection between being unnatural and being immoral. The Christian view of why homosexual relationships are unnatural is that it violates God's purposes for sex and marriage. Being asexual may be unnatural, but I'm not sure being a practicing asexual could violate God's purposes, since Christians believe some people are set apart for singleness and celibacy.
Now it's a separate question whether asexuals should marry, and I suppose part of that might relate to what their purpose for marrying is and whether they intend to have sex once they're married. The Christian view on marriage is not as simple as saying it's for kids or to limit sex to those who want kids, as David puts it. The Christian view of marriage is more about God than about us. Marriage represents the relationship between God and Christ and the relationship between Christ and the church. Sexual union is supposed to be part of that representation, and Paul does say that a husband or wife who denies a spouse sex is doing something wrong, but he also tolerates those who think they need to abstain from sex to have a more devoted time of prayer.
What would he say about those who simply don't want sex? Their marriage would certainly be inferior unless they choose to have sex anyway, but is it just inferior or is it contrary to what marriage is supposed to be the way homosexuality must be if male and female represent the unity and diversity in the Trinity, the submission yet equality of Christ to God the Father, and the roles of Christ and the church. Those can be represented in all parts but the way sex was intended to represent intimacy with God in an asexual marriage. Therefore it's just incomplete. A gay marriage flies in the face of many of those things and therefore is not just incomplete but contrary to the male-female role differentiation that's part of God's plan.
I'm not sure all those who oppose gay marriage will agree with all this, but I think it's the most likely reconstruction of what Paul would say on this, given what he said on other things. Since many of the people who oppose gay marriage are Christians, even if most of them aren't thinking in as detailed a way about the justifications for their views as what I've just gone through requires, I'm not sure David's conclusions will follow.
The moral questions alone aren't parallel, never mind the legal ones. What Christians believe is wrong about homosexuality and asexuality is similar. Something has gone wrong to make someone the way they are, whether caused by biology, social forces, or most likely some combination of both. What would be morally wrong to do, however, doesn't seem parallel at all. If homosexual relationships are wrong to pursue, and homosexual sex acts are wrong to engage in, then that's what someone does on top of being gay. Would it be morally wrong for an asexual to fail to engage in sex acts? I don't know how any Christian would say such a thing.
As for the legal issues, those who oppose gay marriage may well not oppose asexual marriage and be fully consistent if their reasons for opposing gay marriage have to do with marriage's purposes not being limited to what David says those against homosexuality limit them to, and I've explained at least one view according to which that's not the case. So allowing asexuals to marry does not implicitly requiring gay people to marry other gay people of the same sex.
I don't agree with his assumption anyway. Allowing asexual marriage doesn't constitute an endorsement of asexuality. Of course, allowing gay marriage doesn't constitute endorsing homosexuality. Not everyone who opposes gay marriage thinks it does, either. Those who do think it does will need to think about whether we should disallow asexual marriage (but how?) or whether they should stop thinking this is a good argument for opposing gay marriage. On that count, I think David has pointed out something that might influence the discussion in an interesting way.