It's true, Republicans all think that Gay and Lesbian couples are so special, that they are entitled to a special tax rate over hetrosexual married couples. How could they possibly have done this? By making sure gays and lesbians can't be leagally married.
Those of us among the married know exactly of which I speak. There has almost always been a marriage penalty for households where both partners work. Despite this, Republicans are willing to forgo the extra taxes that would be collected from gay and lesbian married couples, in essence providing them with a tax cut that other couples cannot get unless they choose to "live in sin".
I think this is one way to approach the subject, by leading people via their sense of fairness. For example we can pose the question "Do your think that everyone in the US should pay their fair share of taxes?" The answer will be Yes of course. Then you can say that many gays and lesbians are not paying their fair share and something has to be done to fix it. I'm sure they will be ready to sign on the line at that point. Then you go for the closer, that they just signed a petition that would make gays and lesbians have to pay their fair share of taxes by allowing them to get legally married...
It's not the best idea out there, but it does throw a curve at the way Republicans look at the topic. Bring it home to them about something that is near and dear to them, and what doesn't hit home like taxes!
As many have said, we need to start framing the debate...