Well because there is no positive proof you have to look at it two ways. Now if you look at it as if you are astoriod spit, as in evolution, you are just a higher form of animal. So you do as an animal.
But what if you look at it as if you are a devine creation? Well Adam and eve where told by God to procreate but they weren't having sex while being perfect. See how Jesus felt about sex as being perfect. He was the last Adam. No sex no desire. After the fall in the garden now Adam had sex. Lust was the motivating force.
It is a negative response that keeps you and I alive. The 'gift of fear." I propose a" gift of lust." We also have a "gift of hunger." Without those you are dead.
