The hormone Ocytocin appears to be part of the hard-wiring in the female brain that helps women bond emotionally to their sex partners (as well as their friends and children). While both men and women produce this hormone, it is most powerful when combined with the female hormone estrogen, according to some of the articles I link to below.
Even MSNBC posts an interview with an expert who states:
One of the reasons [women don’t understand how men can differentiate so easily between love and sex] is that during sex, women produce lots of oxytocin, a hormone that stimulates a strong emotional connection. As a result, women are more emotionally integrated when it comes to sex. That’s why casual sex and hookups often backfire for lots of women. ...
There is also some suggestion that the bonding effect is more powerful over time spent with a single partner.
Good news for us married women. Bad news for the "hookup culture" as Miriam Grossman notes in an article Dawn Eden cited today:
[UCLA] psychiatrist [Grossman] lamented the "mental health crisis on our campuses." Prozac, she said, is the number one prescribed medication. And Grossman said the rise in prescription anti-depressant use among young women is linked to the rise in the number of women who come to the health center because of sexual relationships.
"We have a problem. We should be alarmed, but we should not be surprised," she said. Young people are influenced by a popular culture that is constantly bombarding them with blatant lies about sex, she said. Grossman said television shows, including Sex and the City and Friends, give a false notion that sex can be divorced from emotions.
It gives the message that sex is "recreational without consequences and that condoms provide good enough protection," she said. Teenagers arrive on college campuses with those ideas and are rudely awakened when they learn that they have been deceived.
"High risk behaviors are being promoted," said Grossman. Because of that, she said, "the number of sexually transmitted diseases has exploded."
The facts are evident, she said, but health care professionals are not responsibly educating young people. Hard science alone proves that bonding hormones are released in a woman’s body during sexual activity. "It cannot be disputed," she said. Grossman said science has shown the release of the hormone oxytocin in a woman’s system during such activity makes her more susceptible to distress, anxiety and depression with a "hooking-up" situation.
Ocytocin - The Cuddle Hormone (Note: Very secular site.)
Femme Mentale (San Francisco Gate--really!)
Neurobiology of Human Sexuality by Tania Romero, Bryn Mawr College