Al Mohler writes about a 1941 letter Tolkien wrote to his son, Michael.
“The devil is endlessly ingenious, and sex is his favorite subject,” Tolkien insisted. “He is as good every bit at catching you through generous romantic or tender motives, as through baser or more animal ones.” Thus, Tolkien advised his young son, then 21, that the sexual fantasies of the 20th century were demonic lies, intended to ensnare human beings. Sex was a trap, Tolkien warned, because human beings are capable of almost infinite rationalization in terms of sexual motives. Romantic love is not sufficient as a justification for sex, Tolkien understood.
There is much more on Mohler’s site.