Bigamy/polygamy and consanguinous marriage among consenting adults will happen. They’re going to get cleared through the courts, using the decisions on same-sex unions as template and precedent. You cannot tell one class of adults they can get married and another not. You just can’t. “It’s icky” is not going to be a defense. “It’s not how marriage has worked through history” is really going to be even less of a defense than with same-sex marriage, because those actually are historical. I suppose there’s a public-health argument, actually a eugenic argument, to be made about incestuous marriage. Good luck finding somebody who wants to make it in a modern American courtroom, let alone listen to it. And it’d be completely irrelevant if, say, a couple of gay siblings wants to tie the knot.
So those laws are gone, as soon as somebody decides to make the case. Somebody will file for the right to multiple marriage. Somebody will file for the right to marry close relatives. God help us, maybe the same somebody. And they’ll win.
But the difference rests with the fact that homosexuality is a mental disease while having two wives is not. I am not Mormon and I only have one wife of near 40 years. When I was young I did have a couple of ladies I was dating at the same time. Not really a big deal.