The more plausible explanation to the Incubus/Succubus is that the person who believes to have had this experience suffers from sleep paralysis.
This could explain stories of alien abductions as well.
This happens when the body begins to enter or leave REM sleep. Body immobilized + brain inbetween wakefulness and sleep = weird things happen which're mistaken as apparations and the like. It happened to me a few times when I was a teen and the experience is unsettling and convincing to say the least.
I am Alpha and Omega, the beginning and the end, the first and the last.