A female demon that comes and visits males in order to seduce them in their dreams. They always want to have sex and suck out energy or soul of their victims. In literature it causes decrease of health or mental state, that later causes death. If we talk about male character - that's Incubus. Probably christian and other religions were trying to find some explanation and shame for wet dreams or something like that and referring to that as a sin. If you wake up with pain in your genitals that's probably a disease, maybe you haven't had sex for a long time, or something else - but definitely not demons fucking you while you're sleeping.
People all around the world, from many different religious backgrounds have experienced demonic entities that attempt to engage with them sexually. There exist many theories as to why and how these beings have come to be. Unfortunately the root of these creatures lay within ourselves and are created from the combination of masturbation and imagination. Through masturbation we create sexual psychic elements which exist on higher dimensions.
On the other hand, we have those who understand energy Vampyrism in all its forms and that many of us apply a code of ethics and morality to our lifestyle. We each seem to be very particular about what we feed on, and how we do it. There are also Sanguine Vampyrs who incorporate blood but I will not get into that because a. I look at it like this, if their participant is willing and not being abused, then no one is in any position to judge adults partaking in taboo acts.
It was a band-aid solution. I was raised in a Christian home, where sex was never discussed, and I masturbated ever since I can remember. When I was 11, puberty started, and added a whole new dimension. I used soft pornography till I was maybe 12, and dirty westerns until I was maybe I was a serious addict until I was 16, when I met my current mentor.