Jonathan Bernstein speculates:
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.