This is highly cultural. And I'd be surprised if, in the US, men "fear" gay men the way you're making out. But as OP clearly stated, there are countries where mixed sex naked saunas/spas are a thing and ogling or other kinds of harassment are not a big issue.

I didn't state the degree. I just mean straight men oppose gay men more than straight women oppose gay women.

Oppose and fear are not synonymous.

In this situation, straight mens' fear is specifically from their insecurity about their own masculinity.

Would you say the same thing about women’s fear of being ogled in an enclosed space while naked?

Ogling for women carries with it the implied risk of rape due to natural differences in physical strength, and women experience rape at a much greater rate than men, so that threat would have higher salience than any equivalent gender-based insecurity.

At a greater reported rate than men, and even if true it doesn’t take away from the fact that men are still very much victims of rape.

So, no, they’re not just worried about it because of “insecurity of their masculinity”.

Ok, so the facts are wrong because you say they are. Got it.

Not everybody is comfortable being naked in front of others generally, and that's okay, but straight men who genuinely fear the possibility of simply being visible to gay men need therapy. The assumption that everybody is just going to be ogling everyone else just doesn't hold up to reality where there's a very long history of people being naked in locker rooms, showers, bath houses, beaches, saunas, etc. while still being polite and respectful of each other. That isn't to say that rude people have never existed, but that is far from the norm and nothing to be fearful of.

I agree, and I didn't mean to imply they fear the sight.

Some gay men pursue straight men and it's obviously wrong.

I guess it depends on what you mean by "pursue". If a person knows that somebody is not attracted to their sex and that person still asks them out or hits on them that's just plain rude. I 100% agree that's wrong. Nobody likes that.

Asking out someone before you know how they feel about you (and the rest of your sex) is perfectly normal and sometimes that's going to result in a gay man asking out a straight man and that's okay too as long as that person backs off after getting a polite decline. I personally think it'd still be weird/rude to do that to a stranger while one or both of you are naked, but it probably does happen in places where people are more comfortable in that setting.