Do you understand Beet's (fortunately fringe) position as necessary in order to be consistent? Because the increasingly prevalent opinion that I've encountered (and which I hold) is that limitation to two partners is qualitatively more important to the institution of marriage in our society, in societies similar to or derived from ours, and in the expected and hoped-for future of our society than is the question of the partners' sexes, and thus it's not inconsistent to put stock in the former but not in the latter.
No no, I think you can legitimately be for adding gays to the definition of marriage and not support polygamy. But to be consistent, you would have to oppose the whole "right to marry who you love" meme. If one takes the position that marriage isn't a right, but wants to include gay couples within that institution, i.e. add to the definition --- but opposes polygamy, then no, there's no inconsistency
