The same arguments made in favor of gay marriage, can also be made in favor of polygamy: who are we to tell people who they can love? Once gay marriage becomes a right in the majority of states, the polygamists will lobby for, and receive, the same rights of marriage that gays have.
If you don’t think that will happen you are DELUSIONAL.
Gay marriage now, polygamy later. Imagine an extended family of bi-sexual polygamists! Five men married to seven women and each other! Holy Vaseline, Batman!