simple, he didn't do it earlier
for some reason the japanese have a fantasy of sitting back and watching/hearing the girl they love get fucked by seemingly every other guy in the universe
but he doesn't make a move to sex her up til she's a complete cumdumpster, but inevitablely he's so much more amazing than the other guys because of his self restrainant.
That is a fucking stupid plotline, even for porn.
It's just fantasy fullfillment for wimpy virgin japanese guys to feel better about not getting any pussy ever, because all his pent up sexual frustration will make him a love machine.
Completely ignoreing the fact that he could of just fucked the girl whenever because she's a complete slut.
Shit like this is what makes me hate the japanese