@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.)