Erik would double cross him now and then when they're on the same side
This is indeed an eternal truth.

Hmm. Well, there were apparently other students for a while, and who knows what kind of relationship he might have had with them. I agree that his new physicality plus the fallout from his one attempt at a relationship with Raven would tend to suggest some celibacy. I think it's really unlikely that he would have had sex, post-Cuba, with a non-mutant. So... not knowing who the students were and so on, it's very possible Charles was the first. (Although Hank would vehemently protest that of course he wasn't, and Hank is very experienced tyvm.)