Sure, why not? If it's between consenting adults, why not?
In the particular case of incest, I would disagree with you Shock. I know the point your trying to make, but the main reason I would say this has the right to be 'policed' is because of what you said later:
The only one I might have a problem with is incest between brother and sister, as that yields a very, very high chance of birth defects.
Polygamy? *shrug* if the women are stupid enough, and the men are crazy enough - I don't really give a monkeys. My only real concern on this issue would be whether kids born to polygamous marriages would recieve proper attention from their fathers...
...then again I saw a program once about a 'normal' family in India who had 33-odd kids!! That would be prefectly legal, but could you claim that each of THOSE children were getting the individual attention they deserved?
Polygamy has been present at one time or another amongst a lot of cultures (including American) and it hasn't caused the 'implosion' of the society. In fact, I really see a polygamous man as just a man who likes to sleep around, but under the 'pretense' of marriage. I'm sure it's not really THAT shallow in particular cases, but I think it's valid to view it to a certain extent in that light in general.
...but at least he has some kind of official ties and responsibility for each spouse and each set of kids - which aren't present for 'legal' casual sex...
MK -if your arguments were purely MORAL (well, at least your ideas of what is moral, and what is not), I would have more respect for them. But these ideas that homosexuals are fundementally abhorent of commitment (any more than your average hetrosexual!), or that allowing homosexual mariages will cause real, practical damage to society -well - let's wait to see the so-called 'evidence'...