It's about time someone asked the question -- why should marriage be between just two people? If two men can get "married," as well as two women, why not three or four or twenty? How are we to know that next year the gay lobby won't say that an ideal "marriage" consists of two gays and one bisexual? If the progressives cannot handle the basic concept of male and female, how are we to expect them to understand mathematics? And what is to stop the progressives from accepting marriages between close relatives (ugh)? Any resulting genetically-damaged children can be aborted--this would be leftist utopia, would it not? Finally, what is there to stop the child molesters from trying to be "respectable" and advocate "marriage" between adults and children? Where will our social experimenters stop? Their only standard of value is that someone somewhere must enjoy it, and "it," (whatever the "it" of the moment is) must be counter to the values civilized people have held for thousands of years. Once you accept that, there is no logical stopping point. The "slippery slope" is at a 90 degree angle.
Marriages within families are still prohibited (even if they are consensual).
The litmus test for polygamy may come from bi-sexuals (since they do not "choose", so the argument goes, to desire persons of both sexes, how could they be made to deny half of their identity?) or a freedom of religion challenge (muslims, mormons, and other religions accept polygamy and some foreigners already have multiple wives).
I've heard homosexuals take a stand against all of these acts saying that they pervert marriage (while still arguing for same sex marriage). Can't close the door, either everyone has these freedoms absolutely and the government can't regulate it or the government has a duty to society to limit the definition of marriage and the nature of relationships.