I don't see why Barney Frank or other gay marriage advocates are any less qualified to fight for monagamy and traditional families than anyone else. Is there other evidence, other than his sexuality which doesn't prove anything that I can see, that Barney Frank opposes monogamy and strong families?
Maybe because his boyfriend, who was about half his age, was running a prostitution ring from his house? (Sorry, I couldn't resist.)
I agree with your basic point, that it's theoretically possible for homosexuals to have family values, but I don't think Barney Frank is the best example to use to make it. And I do think the reality is that homosexuals are, in general, more promiscuous and socially liberal than heterosexuals.