What can science tell us about sex, orgasm and mating? Recent brain science discoveries reveal that sex is governed by the reward center of the limbic system in the brain. A Dutch scientist discovered that brain scans of orgasm closely resemble scans of shooting heroin. Other studies show that after orgasm a "sexual-satiation" neurochemical shoots up, and the neurochemistry affecting libido changes radically. In short, conventional sex can cause a hangover.
Any psychiatrist will confirm that our feelings are projected onto those around us. The natural neurochemical fluctuations that accompany fertilization-driven sex can therefore radically distort how we see our lover, both before and after sex.
Evolutionary biologists, such as Richard Dawkins, point out that the traits of a species are those best suited to pass on the most genes. Unfortunately for our intimate relationships, this end is best served by impulsive passion, followed by decreased interest in the "fertilized" partner, and pursuit of another. As explained above, our brain chemistry is set up to encourage exactly this...an intense "high" during sex, followed by a hangover that alienates lovers.
The work of anthropologists such as Helen Fisher shows that across 57 different cultures, divorce peaks in year four. Studies of hunter-gatherer tribes (who probably best represent our distant ancestors) reveal that the typical mating pattern is lots of passion and romance, and lots of churning in intimate relationships (and emotional distress).
Together these insights form a constellation suggesting that "normal" sex is a culprit in relationship disharmony. Other scientific discoveries point to a solution. If you want to know more, see section Science.