http://www.newscientist.com/dailynews/news.jsp?id=ns9999667 Sex, lies and monogamy At the heart of all long-term relationships lies a fundamental deception - disguised fertility
Exclusive from New Scientist magazine
Women only stay with men for security, and men only stay with women for sex. It's a cynical view of human relationships, but researchers now say it is the driving force behind the evolution of monogamy - and women started it. By offering sex all the time, females in monogamous species disguise whether they are fertile and trick males into sticking around.
In most species, females only have sex when they are fertile. This is because sex takes energy, and carries the risk of disease. But it also means males can easily tell which females are fertile, so they don't waste time on mates that won't get pregnant. [snip]