@pajamawolfie He mentioned in the previous chapter that he's fine with her taking his energy cause he has a ton of it, so my guess is he has way too much of the energy that a succubus needs to the point where it's overflowing and a regular succubus can't handle the amount he gives off, maybe even to the point where prolonged exposure to his energy has the potential to kill a normal succubus. Also, it's very likely that he has an absurd amount of stamina in bed considering what the doctor said, but given he has stated he's never been with a girl before we don't know the actual details of that particular side of the situation. This, of course, begs the question: why would human girls shun him? I mean, his energy will only affects a succubus, and even if he's tireless in bed can't they just make boundaries? Like, tell him to stop after an hour at most or something? That is, of course, unless the idea is once he's in bed he has no restraint. If that's the case, then maybe he could very well fuck them for days on end till his partner dies, whether they be human or otherwise, and they wouldn't be able to stop him without outside help. Hard to say for certain since, as I mentioned before, he hasn't been with anyone yet and we as readers don't have enough information about his situation.