I'm going to say we'd have a lesbian for President before we would a gay man, and this is coming from someone who represents the latter. Reason I say so is because it's a depressingly masculine society we live in and there seems to be a double standard applied to gay men vs. lesbians; what I mean is that society seems more accepting of lesbians because let's face it, it's basically every straight man's fantasy to see two girls getting it on, but the idea of two guys doing the same is viewed as "gross" and "disgusting" and is still pretty taboo.
Furthermore, we know from Hillary's historic campaign that "if we can blast 50 women into space, we will someday launch a woman into the White House."
It's just a matter of when the first female will shatter that highest, hardest glass ceiling, but when she does, it will make it easier for a lesbian to become President. Most straight guys I know are pretty uncomfortable around gay guys but are more than okay being around lesbians; at the same time, straight women led by Kathy Griffin do love their gay guys but probably feel a little weirded out being around lesbians. While females are the majority in this country, I really doubt that it will be a lesbian who shatters the glass ceiling and becomes the first female President so after women have succeeded in achieving that milestone, they may not feel nearly as motivated to vote for a candidate simply because she's a woman. Obviously, right now, a gay man has the upper hand simply because of his gender and the fact that we haven't had a woman as President, but I do think that in 2016 (unless for some gift from God that Hillary challenges Obama in the 2012 Democratic Primaries and wins and then goes on to thump Huckabee or Romney or Bailin' Palin in the general election) we will see a woman receive the Democratic nomination and when that day finally comes that we shatter that glass ceiling, lesbians will gain the upper hand in becoming President.
I guess it's a question of do voters want a President who will make the White House lawn look nice or a President who will give the inside of the White House a facelift, so to speak.