@DigiTrash Eeehh.. Time, place, occasion. Daytime at school might not be the best time and place, and it definitely ain't a good place when it's the first time and in a room that teachers might want to visit at any time.
But for all other comics it's a bit of confirmation bias as well. I've read mangas (even non-H ones) where the MC doesn't shy away when gf offers it up. But once that has happened, that bit of the story is over, they're in a completed relationship. Whereas if he backs off, he can back off in any number of future episodes - indeed it can be the whole topic for long serializations. (And more importantly, it can annoy us that he's acting that way, and we can commit our annoyance to memory.)
At first, I thought that I'd be lewd but not to this extent. The moment I caught a glimpse of those heart eyes my penis fucking ascended past the mortal realm.