Well immediately we can eliminate two of those groups (men marrying underage girls, and people marrying animals) because those unions lack two consenting adults, and therefore they would never be legalized.
But regarding polygamy and incestual marriage, I just don't think it's reasonable to be concerned about that. For one, I'd have to imagine the amount of people who would lobby for such a thing would be miniscule at best, and that's simply because there wouldn't be very many people trying to have these relationships. I'm just guessing, and maybe i'm way wrong, but there's maybe only a couple thousand polygamous and incestual "couples" in the US (compared to millions of gay couples). If they do a little bit of bitching, so what? Let them have their say, and the rest of the country will tell them no
.
But, let's talk hypothetically for a second. Would you support gay marriage if you knew for a fact, that it wouldn't lead to any other non-traditional marriages being legalized?