On the topic of whether liking someone who crossdresses is gay or not, I think it mostly depends on the situation. Like
@summerofblue said, "are you straight because they look like a girl?". Yeah, if you like someone who's the same gender, only when they're crossdressing, it's basically being straight. Like a boy liking another boy only when they're crossdressed. It's not really gay because they're into girls, and they look like a girl.
But if you're still feeling something for them even when not they're crossdressed, then it's kinda gay: You like them for who they are, and at this point you may just be willing to date either gender as long as you like the person's personality - bi.
But in the context of this manga, I dunno. I think he can just be called 'almost straight' or maybe 'mostly straight' because this dude has had no attraction towards anyone other this one dude lmao major respect.
also i'm only basing this off of morals not facts pls don't take what i say as a fact its only an opinion