The Hard Problem.

I am sick and tired of hearing anti-gay rights politicians spouting off about how marriage is just between a man and a woman. Let's go ahead and ignore all of the stupid arguments that we've heard about how the purpose of marriage is child-rearing. What about an infertile couple, stupid?
Here's an even harder problem: what about a trans-gender couple? Can they marry anyone? Or can they marry no one? I'd love to know the answer .