I dont think he needs to have sex with them to get to there heart but by having sex with them its gets them to relax and feel good which makes there heart taste better. For example in chapter one when they were attacked by two men and one of them ran away once they saw what the incubus true form and manuel started to shot at him and started scaring the crap out of him and the incubus stopped manuel from hurting him more because if the heart is in a frightened state it won't taste good so thats why it's best to come them done but this is just what i'm thinking from the many times I've seen the incubus feeding on others.
Just what happened? I thought he was an incubus and that why he had sex and fed on humans but turns out he's not? I'm so confused... ┗( T﹏T )┛