As for me (also a gay man), I definitely do notice when I find a woman pretty… it’s just not as automatic as it is with a guy. For most of my teenage years, this was enough to convince me that I was straight
and even for a little while after that, that I must be bi. But it was a different kind of attraction… I’d just sort of internalized the idea that if I find someone pretty, that must mean I’m into them…
(For that matter, I can also sometimes notice that a man looks very good but is nonetheless not my type
)
Well, it depends what change you’re referring to, really. It’s all rather complicated inasmuch as things happen at different times in different places, and it’s not always linear… for example, a lot of times increased visibility then leads to a backlash. There’s also a lot of unexpected bits in history that most people might not realize… like, there was a gay subculture in late 1700s Britain called the “mollies”… the French Revolution got rid of sodomy laws… in the US, the Prohibition period actually tended to benefit gay subcultures (because all bars were illegal, so gay bars were on a similar footing to the other ones)… there was a lot of progress in Germany until the Nazis cut it off… etc…
I think this is what I was trying to get at with my comment, but expressed a lot more succinctly, so thanks 
The thing is that discrimination can come from very innocuous-seeming places… it can come as a big surprise. And if you’re primed to expect that bigotry only comes from the most extreme sources, it’ll come as an even greater shock, and it’ll be easier to make excuses for them.
(And, well, ignorance does happen. If a person only ever has heard stereotypes about gay people, it’s not too surprising if they’ll believe them
but then, those are also the sort of people who would be likelier to be able to learn when they meet someone from that group and realize that those stereotypes were wrong
)
“Should,” yes, but a lot of times people just don’t
and I think it should be possible to show such a stubborn character without normalizing them for it. Since it’s the kind of person that many people have to deal with in their lives, it’s prime fodder for narrative treatment…
(And I think there should be ways to show that a character’s wrong without the character realizing it
)
And, if you’re using mobile, and what to quote additional posts, but the quote box has filled your whole screen, you can click on the three horizontal lines in the upper right to make the quote box small again, so you can scroll to another post and quote that one 