2 factors.
One, we are out of the closet. Gay men are everywhere and Str8 men know this. There was a time when the last thing one would think is another guy is "gay", because the social model of gay was so horrific and stereotyped anyone without a lisp, swish, or tentacles on their forehead was viewed as being straight. -Now, almost any guy next to you could be gay. So there's greater reason to 'hide the goods.'
Two,
In the minds of most straights, HIV -for the most part- still equals gay. All forms of physical consequences -or lack thereof- have an impact on social norms. Throughout history periods of sexual freedom have been followed by pandemics followed by a repressive backlash in social norms. On the flip side, advancements in contraceptives have completely changed social norms about the body. You would not believe the kinds of norms people put up with in America before the pill. It is like a time from another Universe compared to present day.
I wouldn't say its entirely "homophobic", because I don't think those guys are really thinking in terms of men AROUND them, but more of themselves. It is a desire on their part to NOT be an object of attraction. There is still a strong force in society for men to be 'neutral' and for women to be attractive. And we're talking a world view of heterosexuality here.... As the line goes in the movie Parting Glances, it's true: Straight people are so narcissistic that they believe 99% of everything is about them...
So we're in a strange flux. On the one hand, attitudes towards gay men is being more socially accepted. But the perception of straight men that the consequences of being an object of desire is leading to a more Victorian attitude of the male body. (Even with the explosion of male imagery, thanks to the internet.)
Something is being worked through in the psyche of the modern male. And there are forces at work that are at complete odds to each other. It's a divide, perhaps parallel to the political divide that is deeply splitting our society. We don't quite know how to be at the moment...
(I hope this doesn't read as bigoted by any str8 members on board... just commenting on attitudes natural to human beings.... Most everyone is narcissistic enough that they think 99% of what matters is all about them... gays included.)