I think the only thing they couldn't justify is pedophilia, using the libertarian notion that childen cannot consent. I suppose they can say, well, no we won't push for bestiality because animals cannot give consent, but would animals have consciousness of being violated? I guess they could say it's animal cruelty, but what if the animal is not harmed? I suppose they could say, well, but the animal wouldn't enjoy it, but how would they know that?
But you're right, how can they so blithely mock conservatives' concerns about ultimately normalizing incest and polygamy.
And why do they mock religious polygamy? Would they have a problem with polygamy if it were a bunch of dudes? Or how about a whole gang of gays and bi's of both sexes? How do they logically say that will never be the next step? When they cast aside the traditional definition of marriage, where do they get the basis for the one man-one man/one woman-one woman gay marriage paradigm?
"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." - Manuel II Palelologus