I'll get to the second half of your post in a second, first let me clarify something, What I said has zero to do with gay rights or gay marriage.
The island I'm talking about is all about mental and social perception. Even the people who have issues with minorities and women don't have the same kind of issues that they have with gay people. The issues people have with minorities and women are social issues, the backbone of the prejudice against gay people is religious issues, that is why it's different. Religious conviction is a stronger tie to break then social convictions, so ethnic minorities and women have more of an opportunity to sway voters to their side. For millions of voters who hold religious convictions homosexuality is a sin, and those people cannot imagine voting for a gay president.
You speak of the speed that gay rights have achieved, well the speed is actually part of the problem. The people who used to call gay people F**gs on a regular basis haven't gone anywhere, they are still among us and they still vote, those people are smart enough to know they can't be outwardly bigoted but there isn't anybody watching them in the ballot booth. Black people have had decades of white teenagers listening to black music, little white kids wearing the jersey of their favorite NBA player, and middle-aged white women getting life advice from Oprah, these things along with dozens of other things have made it acceptable to see black people as well.......people. Women have had a similar (but still kind of diffrent) road to mainstream recognition as contributors to society. Despite all of that, the extreme racism black people still face, and the rise of the #MeToo movement shows that there is still a long road ahead of us. That long road exists for gay people as well, but they haven't had the same time period of mainstream acceptance. So all I'm saying is don't be fooled into thinking we are more progressive than we actually are.
If all else fails just remember we have had 50 years of the feminist movement yet Donald Trump, a man caught on tape saying he sexually assaults women still got 41% of the female vote (52% of white women), so we still got a lot of work ahead as a society.
Now as for who is the strongest candidate to beat Trump I have to go with the polls, Biden seems like the best bet (even if he isn't my choice), and if he picked Stacy Abrams as his running mate (as rumors are reporting) he would have a solid chance in 2020. Personally, I'm leaning more towards either a Bernie Sanders/ Nina Turner ticket followed by an Elizabeth Warren/Andrew Yang and then an O'Rourke/[woman to be named later} ticket.