When arguing about gay rights with people in the past, I've personally raised the whole polygamy/bigamy/incest issue and I've had god only knows how many people shrieking that it's a specious comparrison. I've never had anyone actually explain why the issues aren't comparable, but it always ends up like loosing a bull in a china store. They seem to have a lot in common. Both polygamy and gay marriage deal with changing the common perception of marriage as defined as one man and one woman. How can you allow one and still find it justifiable to restrict the other from a legal standpoint? Wouldn't the acceptance of either one sort of open the door for the other? The only conundrum I can think of is it would be an absolute mess for things like power of attorney and inheretance rights if you leave behind multiple spouses when you croak, or for benefits that usually cover families. Somehow I doubt that is the crux of the arguement when polygamists don't want to be lumped in with gays and gays don't want to be lumped in with polygamists.
Even if you consider incest, it's sort of odd that we would criminalize it if polygamy was decriminialized and gay marriages recognized. The old arguement was that close familial interbreeding would cause genetic abnormalities, yet there aren't laws that prevent others with known genetic disorders from marrying or breeding. In fact, there are genetic counselors nowadays who couples can go to to screen for common genetic ailments that might result if they decided to have kids. While those counselors advise individuals on what the risks are, there is no law or authority that could stop a couple from having a child even if it were known with 100% accuracy that it would likely develop tay-sachs or huntingdon's or whatever. While incest turns most people's stomachs, is there really any legal reason to criminalize it when we don't do so for other potential genetic timebombs?