I mean, I wouldn't even say Himari is straight up gay. I think his physical attraction towards Gaku stems first from an emotional connection, which is something he's just never cultivated with a woman because of his "fear".
Then with Gaku.... I'm friends with multiple men that identify as "gay" that have slept with women in their younger years. If I was to go off what these friends have told me, they are able to sleep with women, but they just don't really enjoy or feel it which is basically what Gaku says when he talks about his middle school life.
There are many gay men and women that have sex with the opposite gender and even have children before realizing they’re gay, especially when it comes to older lgbt individuals that grew up in a time where being gay was still taboo. To put it bluntly sex can be pretty quick and the body can be pretty reactive at a young age to where someone can be stimulated enough to have sex even if there isn’t any real attraction there.
gaku had sex in middle school? i know there are a lot of tweens who have sex really early (too early imo) but i feel like the author made gaku lose his virginity as early as middle school just so that he would be "experienced" enough to be able to teach the inexperienced himari in high school. idk, considering gaku had not really tuned into his true sexuality yet and was still unsure even when he entered high school (after having sex ) its surprsing, to say the least, that he found himself able to to have sex with one of his girlfriends. so is he not completely gay? because with himari its clear he isnt aroused by women and wouldnt be able to have sex with even if he tried