Sexual behavior was traditionally constrained by the requirements of survival—you needed a certain number of children to help till the fields and avoid starvation, so people subordinated personal desires to necessity. Today the risk of death is removed, so people do what they want—and "what they want" is therefore subject to greater selective pressure.
Your picture of the human condition predates the agricultural revolution. That doesn't mesh with your idea that evolution happens in the blink of a couple generations.
