I hate it when people use the bestiality marriage example because that IS over the top. I think it's a red herring because it obscures the fact that polygamy is right around the corner. The muzzies will sue if they don't get the right to four wives... it will happen here too. The leftists will say that you can't point to anything wrong with it, and if you can't bring God, morality, or religion into it, then I suppose there isn't.
OTOH, at that point I become an enemy of the whole concept of civil marriage and any tax or any other benefits that accrue to married people. It's not really fair for me to subsidize marriage in the first place, and I certainly don't want to do it for those who are trying to destroy the system. MAYBE a tax credit per dependant, but I'm not even so sure about that.