Humans lead the pack:
So why don’t monkeys and apes masturbate even nearly as much as humans? It’s a rarity even among low status male nonhuman primates that frustratingly lack sexual access to femalesin fact, the few observed incidents seem to be with dominant males. And why haven’t more researchers noticed such an obvious difference with potentially enormous significance for understanding the evolution of human sexuality? After all, it’s been nearly 60 years since Alfred Kinsey first reported that 92 percent of Americans were involved in masturbation leading to orgasm.
The answer for this cross-species difference, I’m convinced, lies in our uniquely evolved mental representational abilitieswe alone have the power to conjure up at will erotic, orgasm-inducing scenes in our theater-like heads … internal, salacious fantasies completely disconnected from our immediate external realities.
If Aquinas were working today, masturbation would not violate "natural law" for homo sapiens; it would exemplify it. I can't remember discovering any act quite so spontaneous and quite so wonderful as masturbation. If that impulse wasn't part of my nature, what was? In my teens, it was a marvel that God had given me such an amazing source of pleasure, fantasy and excitement. And portable!
I also remember, being the good Catholic boy that I was, pondering whether it was obviously evil to the reflective conscience. The question seemed - and seems - ludicrous, etching within me my first dissent from orthodoxy, on the basis of the doctrine being self-evidently stupid. (The second was trying to believe that the Virgin Mary was physically whisked upward into the upper atmosphere - and that under papal infallibility! Strike two.)
This much we know: every priest masturbates. Because they are human beings, and the sperm has to go somewhere.