I Am thinking that many more moderate Democrats are not okay with gay marriage and might be shocked if their congressman or senator approved. The Democrats in many parts of the country put on a more normal, conservative façade when they are back home so it might shock them to learn that their average Joe kind of congressman is really more liberal than he let on.
Of the 232 Democrat Senators and Reps, I’d be shocked if you could find a half dozen who would answer the question NO.