"Copy this sentence into your livejournal if you're in a non-same-sex marriage, and you don't want it "protected" by the bigots who think that gay marriage hurts it somehow."
I have a variation on this that I feel sums up my point-of-view:
"Copy this sentence into your livejournal if you're in a non-same-sex marriage or relationship that you feel would be threatened by those who oppose same-sex marriage."
This could be anything that's out of the ordinary. In my case, my relationship is with a man more than 30 years my junior. It could be a relationship with mixed genetics between races that were once deemed unsuitable. Or a mismatch of religion. Or being poly. Or practicing some form of variant sexual practices, that were possibly illegal until fairly recently. (Hey, I'm practicing non-marital sex: they threw people in jail for that, in some places, in the mid-20th-century.)
Go for it, folks. Do your rights to engage in the relationship or marriage of your choice feel threatened by these folks? I know mine do.