The idea of using a spell to control women in order to sleep with them is not one I really like, but it obviously has to happen to explain Elara's powers. What would be great (in my opinion) is that, as the story progresses, Ethan realizes that being linked to the succubus can kill him in the long run and to break the bond he needs to sleep with several women (becausen Elara is already very powerful), but it has to be done under the same conditions as with the succubus, cumming inside them and they do it willingly.