It could work in one of several ways. They could be completely repulsed by sex and constantly starving themselves, which would obviously be a great point of conflict for their character. If they weren't also aromantic, they could hang around humans for the romance and then have sex to please the human and feed themselves. But then also pain because that would make the relationship painful. Or, they could be disinterested/indifferent and just forget to feed themselves. All the other succubi/incubi are all like, "ye, food and sex" and they just don't get what all the hype is about. And when they go about trying to seduce a human they're all bored and like, can I go home now?