A lot of people will at a conscious level think, "I'm tolerant, I'd definitely vote for a gay president," but subconsciously they'll be a little uncomfortable with it, and
if they're not sure who to vote for, this'll tip the scales. This subconscious effect is best captured in neighbor polling, where "your neighbors" IMO ends up being a proxy for "your community's collective subconscious":
That gap grew even wider when voters were asked whether they thought their neighbors were ready for a gay president. About a quarter of respondents answered affirmatively, while 46 percent said their neighbors were either definitely or probably not ready for a gay president. Pluralities of both Democrats and Republicans, as well as independents, all said they did not believe that their neighbors were ready for a gay president.
This might bode poorly for him in a general election, though I doubt that a lot of people are
wavering on whether to vote for Trump or not -- most people either love him or hate him --, and the LGBT thing is an effect which pushes most people only slightly. Also, any negative effect will be counterbalanced to some extent by some LGBT people being actively driven to vote for him.
....
If all that was true, how did Hillary run at all? By avoiding this delicate issue?