Everything in our world has a definition. In these definitions we logically describe the most important features and specification of those things. If the main and let's say the definitive properties and specification of anything changes, it might be not the same thing. For example, we describe a knife with sharp blades, if the blades of the knife is not sharp, we can't say knife to it. This concept is the concept that Aristotle believed in.
So the main property of any living body is being alive. If this property is taken from a living body, it will not even exist. I mean that a dead cat is not a cat anymore. It means that if you make some living body dead, you changed on of its properties so that it loses its entity. If you review the history of "Evolution" you can see that the most important intention of every thing in this universe is to exist. And for a living body existence means being alive. Because there is no immortal organism, the existence of a specie means its survival. and the most important goal of a organism would be generating the next and next generation.
Generating the next generations is probable just through sexual relationships. And this is what we call "SEX". That's why through the evolution the sexual attraction and sense became the strongest instinct in living bodies. And also human beings. If you want to treat with a human you must always consider his/her sexual demands. This the most important instinct of anyone.
But all the religions want to fight with this strong instinct. But let me say "they're all losers!"