This must be a joke right? The very first girl in the original GTO run, the one that duped him but he helped later was already okay with being with him and he turned her down because of whatever reason and she vowed to wait for him (but was never seen in the story ever again). Not to mention the original teacher he was paired with, the home-stay ladies in Shonan, KANZAKI? All of whom are legit head over heels for him and he turned them down due to moral reason or whatever.
Now this random highschool girl promise to have sex with him and he goes for it?
I know this can be an act and he obviously won't go through with it, but he never acted this way before prior to the fact ever. He may be a perv but never when it comes down to doing X for sex, especially to a student.