If you want to get a straight guy to do anything, you have to call his manhood into question. (Whether that manhood is metaphorical or the literal schlong swinging between his legs is a matter of context.)
This video from CollegeHumor does a pretty good job of calling the bluff of dumb, homophobic straight men everywhere who oppose gay marriage: Want to deny us equal rights? Okay, bitch, then we’ll marry your girlfriend. Because guess what? They like us better. No, I’m sure she loves watching you play Halo 4 all day. I bet that just leaves her in a puddle of anticipation.