@Mitsuboshi I personally think that since homosexuality is accepted, incest has no logical reason to be rejected either, at least assuming they don't reproduce and especially if said incest partners are same sex anyways. Being inherently unable to reproduce is no longer an argument against marriage after all. It's one thing to be unable to reproduce due to an ailment or something, but even in good health, homosexual couples can't reproduce and incestuous couples are largely advised against reproducing. Arguably, even the potential for mutant babies is a bad argument because preventing reproduction due to inferior combinations of genes is arguably eugenics, not that I agree with that reasoning.
Sure, you could say incest is gross, but people say the same about homosexuality, so that's a bad argument too. Unless you're also against the concept of homosexual marriage too, then at least you're more consistent than someone opposing incest but being okay with gay relations.