I don't see what gay marriage will do in terms of added value, other than appease the equal rights of one group in defiance of another group.
Polygamy rights are ignored, or like you point out, cousins can't marry.
Equally allowing marriage for these groups adds nothing to the value of marriage as an institution so why bother?
Personally I see marriage failing at the same point Christianity falls out of sight, gays will then have the right to marry at the same time nobody bothers getting married anymore because it's meaningless. Whether this contributes to that situation or not I don't know, but I doubt it helps it.