Indeed. Wether we're talking about incestuous relationships, polygamy, polyamory, bestiality, whatever, it's all the same now that we've redefined (or more accurately, "undefined") marriage to now include homosexual couples.
If the argument for traditional one-man-one-woman marriage is only an ambiguous moral argument open to interpretation, what possible basis can there be to exclude the recognition of any other sort of "relationship" that someone may dream up?
That the Left and the gay community so casually discards this obvious consequence is astounding, and intellectually dishonest.
You know, that's a good question. All states that restrict/prohibit kinship marriages do so citing they're protecting the public's general health, safety, and welfare. Why can't they cite the same prohibition excuses for gay marriage (since it's known to be detrimental and costly) and just leave it at that?