All I know is some hazy stuff about oxytocin and vasopressin. I have no idea if any of this is useful to you:
“Plasma [oxytocin] levels increased during sexual arousal in both women and men and were significantly higher during orgasm/ejaculation than during prior baseline testing.”
http://www.ncbi.nlm.nih.gov/pubmed/3782434
“Laboratory investigations into hormonal release during human sexual activity show that oxytocin is necessary for subjective pleasure during arousal and orgasm in both sexes, while vasopressin is released only during male arousal. Vasopressin is suggested to have the opposite effect on female sexuality, and to impair women’s interest in sexual contact. As oxytocin and vasopressin are regulated by oestrogen and testosterone respectively, and are sensitive to developmental hormonal experiences, they could provide a link between biology and culture.”
http://www.ingentaconnect.com/content/routledg/csmt/2004/00000019/00000004/art00005
“These results suggest that oxytocin, released with mating, may be critical to formation of a partner preference in the female prairie vole; this contrasts to vasopressin, which appears to be more important for pair bonding in the male of this species.”
http://psycnet.apa.org/?fa=main.doiLanding&doi=10.1037/0735–7044.109.4.782
So that was just a little bit of googling—but it looks to me like there isn’t actually a clear answer—just some speculation about different hormones acting somewhat differently in the two genders. Maybe the idea is women have more endogenous oxytocin so form stronger bonds? That seems like a reasonable guess to me but I don’t have anything to back it up with. Read the wikipedia pages on each and go from there; you might be able to find more than I did.
@BronxLens: Thanks :)