First serious presidential candidate with a chance at election who is openly gay--what year, if ever?That's a tough one. Let's do some perspective. As far as I can remember, Tammy Baldwin in Wisconsin this year is the first Senate candidate with a serious chance at election who is openly gay. Baldwin was previously the first openly gay or lesbian non-incumbent elected to the House of Representatives. That was in 1998; I believe that Jared Polis was the first openly gay man non-incumbent elected to the House, and that wasn't until 2008. Has there ever been an openly gay or lesbian gubernatorial nominee? Not that I recall...I don't remember anyone who was a serious contender for a nomination, although I'm sure someone will remind me if I've forgotten one.
So the pipeline isn't there right now. Does that mean there are real barriers? Hard to say. The Wisconsin Senate race will surely be interesting. At the very least, it seems quite meaningful that Baldwin was able to clear the field in the primary as far as strong candidates are concerned, suggesting that Democratic Party heavyweights were happy to rally to her.
Let's put it this way: would it be a real surprise if Baldwin wound up a legitimate presidential candidate as early as 2016? No more than Barack Obama was a surprise in 2008 (of course, she's at best a very slim favorite to become a Senator, so it's certainly possible her political career will wind up fizzling out very soon). Would it be a shocker if Polis moves up the ladder to a Senate seat and eventually launches a presidential run? No, I don't think so.
Overall, I think that there are only light barriers right now in the Democratic Party, and anyone who wins a nomination is automatically a viable general election candidate.
On the other hand, we're not talking about a very large population to begin with. If issues of particular interest to their community recede in the future, it's hard to know whether gays and lesbians will be disproportionately drawn to politics or not. And even light barriers, if they work throughout the system, could make it significantly less likely that a viable candidate will emerge. So it wouldn't shock me, either, if it doesn't happen for decades.