This is going to be a tough one. I read this book as a preteen and I distinctly remember it being uncomfortable and inappropriate for my reading age.
It was a fantasy book involving a young male protagonist who is transported to another world (possibly through a portal on a beach). In this world, he encounters a female spirit that becomes sexually involved with him / forces him to fall asleep. When he wakes up the next day, the young man is now a fully grown, strong adult. Other details that I remember include "giants" of some kind as the villains of the book, with reference to women getting ripped apart by having to have sex with the giants and/or give birth to giant children. Also, there was a scene where the protagonist eats the flesh of a female demon to bring her under his control, and a scene where the protagonist is aroused by a statue and jumps into a pool of water to hide it.
I believe the book was written by a male author and had an orange cover. I think it may have been the first in a series.