It's not "gay" marriage, it's SAME SEX marriage, which means that it's discrimination based on gender when men are allowed to marry women and I - a woman - am not. My sexual orientation is really quite irrelevant to the legality of marriage since homosexuals can and do get married legally now. They just currently have to marry the opposite sex.
The point is, when you get a marriage license, they don't ****ing ask you your sexual orientation. They do, however, ask you your gender. So, whatever orientations it is that you're referring to (and polyamory isn't an orientation) CAN get married right now. No one is stopping them.