Following up Kyle Hale's lead from Beth Seelig... (I don't have enough reputation to comment there)… it seems to be an old saying in psychoanalytical circles.
"In a House of Dreams and Glass: Becoming a Psychiatrist" by Robert Kitzman (Random House; 1996) has a very similar quote on page 54. It's snippet view on Google Books, so hard to tell the context, but it's a quote within the dialogue of some story about therapy.
"She's going to try to hook on to you and to get
under your skin. I once had a supervisor who used to say that
'Everything in therapy is really about sex except sex, which is really
about aggression.' I smiled but was puzzled. I couldn't tell how
wholeheartedly he accepted this proposition."
Susan Putnam has the same wording in "The Talking Cure: The Science Behind Psychotherapy," (Putnam's; 1997) on page 160. She implies that it's an old chestnut in Freudian circles. (Also snippet view)
"Indeed, the importance with which sex and aggression are viewed as
motivators of human behavior by psychoanalysts is captured by the
saying, "Everything is about sex except sex, which is about
aggression.' I think of the Beast Within as a wolf, perhaps the wolf
from the story of Little Red Riding Hood..."
There is also Henry Kissinger's famous quote that "Power is the ultimate aphrodisiac" which may have interbred with this one.