One common misconception about oxytocin is that it's a female hormone.
That's understandable, because for a long time, scientists only knew about oxytocin's role in labor and breastfeeding.
We now know that oxytocin is released in a variety of social situations, including during sex and orgasm. The working theory is that release of oxytocin at climax connects the pleasure of sex with the particular sex partner.
This study is the clincher. A team of scientists measured the plasma levels of oxytocin in 13 women and 9 men while they masturbated to orgasm.
Plasma [oxytocin] levels increased during sexual arousal in both women and men and were significantly higher during orgasm/ejaculation than during prior baseline testing.