"Presumably the masculinization of the brain shapes some neural circuit that makes women desirable. If so, this circuitry is wired differently in gay men. In experiments in which subjects are shown photographs of desirable men or women, straight men are aroused by women, gay men by men.
Such experiments do not show the same clear divide with women. Whether women describe themselves as straight or lesbian, “Their sexual arousal seems to be relatively indiscriminate — they get aroused by both male and female images,” Dr. Bailey said. “I’m not even sure females have a sexual orientation. But they have sexual preferences. Women are very picky, and most choose to have sex with men.”
Dr. Bailey believes that the systems for sexual orientation and arousal make men go out and find people to have sex with, whereas women are more focused on accepting or rejecting those who seek sex with them."
"The best evidence for a long-term attachment process in mammals comes from studies of voles, a small mouselike rodent. A hormone called vasopressin, which is active in the brain, leads some voles to stay pair-bonded for life. People possess the same hormone, suggesting a similar mechanism could be at work in humans, though this has yet to be proved."