I've begun to wonder if humanity will ever change. Will we continue to argue over civil rights every time the world changes a little bit?
In the 20th century we argued over women's rights and racial rights. In the 21st we're arguing about homosexual's rights. I'll predict that in the later part of the 21st we'll argue about genetically modified human's rights, in the 22nd we will argue about robot rights and in the 23rd alien rights.
Now you're thinking I'm a crazy person who just wants hot alien robotic sex. Maybe...but I'm trying to make a point. And besides, the issue is not about sex. It's about love & marriage. Otherwise people would be up in arms about blow-up dolls.
In response to something like gay marriage, people against it often bring up the slippery slope argument. They ask what prevents someone from marrying their sister or a horse? Well, the first is actually genetically damaging. But if you weren't to have children one could argue for it, though it seems psychologically disturbing to me. The second (the horse) isn't sentient or able to communicate at the level necessary for meaningful love or marriage.
So, if the requirements for marriage are sentience and the ability to communicate then there's no reason people won't be marrying robots and aliens in the future. And there's no reason homosexuals shouldn't be able to marry now.