It's not only same-sex couples who feel that way. I'm in a "straight" relationship, and don't intend to get married until all my friends can too. To me, it would be like eating at a lunch counter while my black friends waited outside.

You could say the same thing about eating or having a home, I suppose. But those are very complex problems that I don't feel are going to be solved by just giving up all my stuff and becoming another homeless starving person. In contrast, discriminatory marriage laws are a simple problem to solve: change the laws.