Scientific staff insist that after puberty, adulthood, a man does not attract the woman of his close friend. It happened in the course of evolution. The man who invaded the woman of his fellow tribesman, was considered an outcast. And so it was at all times. Thanks to the friendly relations between men created groups, tribes, Empire State. That is why, at the genetic level to lay: a woman neighbor - danger. Moreover, in the presence of a lady friend testosterone levels in the blood, which is responsible for the attraction falls sharply, thus preventing any desires or intentions.
Women responded to this discovery unanimously testosterone falls only because most men do not want to go so florid by conquering women. They deliberately avoid such challenges. In reality, the pretty wife of a friend - an excellent opportunity for fantasy.