I interpret it as Incubus being not a demon that has sex with women in their sleep, but a condemnation made up by religious people for wet dreams.
If the wifey has a little happy dream about the mailman one night, they want her to wake up extremely disgusted with herself, thinking that she has sinned.
Pastor Billy Bob: Mrs. Smith, you have been visited by the Incubus demon three times this week. God will become testy if you continue to try his forgiveness.
Mrs. Smith: I'm terribly sorry, Father...it's just...we got a new poolboy and....