I've made that point too. Heterosexuals have never had more rights than homosexuals. I also don't have the right to marry my sister or a dog. Neither do they...at least so far.
I think that door is wide open. Polygamy will probably be the first step. While there are men who have managed more than one wife and even done "reality" TV shows about it, the day when a woman will only be happy with multiple husbands is eminent. My guess is the first foray to test this will be a bisexual woman or man that wants to marry one of each sex and proclivity.