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.
Also, keep in mind that only a few states actually matter:
Florida
29 EC votes
Cook Political Report: Toss up
lgbtmap.com LGBT per capita: 4.6%
lgbtmap.com rating (higher = more pro-LGBT laws): 7/40.5
Pennsylvania
20 EC votes
Toss up
LGBT per capita: 4.1%
lgbtmap rating: 15
North Carolina
15 EC votes
Toss up
LGBT per capita: 4.0%
lgbtmap rating: 3.5
Arizona
11 EC votes
Toss up
LGBT per capita: 4.5%
lgbtmap rating: 3.75
Wisconsin
10 EC votes
Toss up
LGBT per capita: 3.8%
lgbtmap rating: 13.5
------------
Michigan
16 EC votes
Lean Dem
LGBT per capita: 4.0%
lgbtmap rating: 14.5
Minnesota
10 EC votes
Lean Dem
LGBT per capita: 4.1%
lgbtmap rating: 28.25
New Hampshire
4 EC votes
Lean Dem
LGBT per capita: 4.7%
lgbtmap rating: 18
Maine at large
2 EC votes
Lean Dem
LGBT per capita: 4.9%
lgbtmap rating: 27
------------
Ohio
18 EC votes
Lean Rep
LGBT per capita: 4.3%
lgbtmap rating: 1.75
Georgia
16 EC votes
Lean Rep
LGBT per capita: 4.5%
lgbtmap rating: -1.5
Iowa
6 EC votes
Lean Rep
LGBT per capita: 3.6%
lgbtmap rating: 14.5
Maine's 2nd
1 EC vote
Lean Rep
LGBT per capita: 4.9%
lgbtmap rating: 27
Nebraska's 2nd
1 EC vote
Lean Rep
LGBT per capita: 3.8%
lgbtmap rating: -0.5
From that initial data, maybe these states would be pushed a bit toward Trump due to the LGBT issue: North Carolina, Ohio, and Georgia? I'd like to see per-state polling on this, especially with the neighbor question.