Mammalian evolution. The opportunity cost of mating is generally much higher for females, so females tend to be much more choosy and guarded sexually, whereas males are much more fixated on mating with everything that moves. There are species where the female will eat the male after mating, and the male will still hump her.
Men are generally much more horny than women, a that that has throughout history caused lots of problems. Wars have been fought, tribes eradicated, and villages burned down, because men were horny. Its because of genetics that we men are slaves to our libido, or at least much more so than women. Not a pretty answer, but a truthful one.
The good thing for us men, is that women are programmed to like us, even if we strictly don't deserve it all that much.