Its biological my dear, eg Human nature is for guys to want to procreate and spread their seed as many times as they can, Thery are on the planet to multiply the population and nature says have a go anywhere they can. Its only religion and politics that say be faithful to one person, nature says do it anywhere with anyone as and when you want to, both to male and female, have you witnessed the animal kingdom, well men are the same, for real, the only difference its up to females to give the go ahead or not, and some aninmals say no at times, take a look at a nature programme on tv and you will relate to loads of how they reproduce. By the way its only the ape family and humans that do it for pleasure eg intimacy ( or security and love if drunk)