Most succubus stories acknowledge on the first page that they live by fucking many men for life juice. Mc gets them by fucking them really good, or treating them like a person, or the succubus is a futa and turns the dude into her bitch or pumps him with so much cum that it transforms him into her waifu or another succubus.
So, the reader self inserts as Chad, a kind person, a woman, a sub, or a secret nymphomaniac. Not an incel.
Demon girls and succubuses are usually used interchangeably in hentai. If not, they turn the male into a ghoul or kill him.