I don’t think it’s necessarily unrealistic,
@Kirin-kun. Try looking at it like this:
Different people and different relationships work in different ways. People can and do manage to wait to take their relationship further physically speaking through circumstances like that. I think that for their relationship this actually makes the most sense, that he was tempted and then for her sake made a conscious choice not to. They're the sort of couple that would need to talk about doing it first; if he’d continued then it wouldn’t have been like him nor consistent with their relationship so far.
As for the “forever,” well. The sentiment isn’t usually realistic, but that’s definitely something people in real relationships say with varying degrees of truth. 😂