An Old idea very popular in alt.sex.cthulhu, amongst other places, you Summon a demon thru Evocation then you try to make it have sex with you, this of course led to a horde of internet porn stories on the newsgroup. Fun idea, repeated in a lot of hentai and books like La Blue Girl. Fun stuff, except that usually the Gods/demons you have sex with bear some ill will towards you over it, and then turn you into their intergalactic sex slave or do something rather rash over it.