I don't get it. I really don't.
Fine, I understand if you're a Muslim, and pre-marital sex is not allowed, I'm fine, I can accept that.
I can also accept if he was being faithful to some other girl, and he promised to do no one else but she. To be frank, he might have made such a vow somewhere in the manga, but I don't remember he making any such vow.
So essentially, he's not bound by his religious beliefs, nor by any commitment to any other living person. On top of that, he lives in a society in which such behaviour is accepted. He's not leading her on, and neither is she related by blood. So why the eff is he so hung up on not banging her?
Speaking as a guy who was oblivious to all female signals in his youth, I dare say that MC is just fucking stupid. We get a cliff-hanger now, and the author will just derail the situation with some dumb shit. Never have I ever felt like wanting so much to hold a lit candle under someone. Author is just plain lucky I'm not in Japan.