Honestly, it is a bit of a legitimate concern that, in a lot of states, two people have to be a man and a woman to get married, and that in any other case, it's illegal because people are "uncomfortable" about it.
Getting married doesn't sound like a big deal on its own, but I'm pretty sure there are a lot of legal benefits to marriage that you wouldn't have if you and your homoloveual partner were just "together".