Our relationship with sex is unique in the animal kingdom. This is what differentiates us from other species.
Our relationship (obsession) with sex is what separates the human mind from the mind of other animals. We have effectively decoupled the act of sex with the act of reproduction. The primary function of sex is not to reproduce. In the UK only 0.1% of all heterosexual acts result in the fertilization of an egg—not including non-heterosexual acts or acts of masturbation (were they to be included the percentage would be significantly lower). Enjoying sex for the sake of it is a unique function of the human mind. It is what separates us from animals, who fornicate to reproduce.
There are many other specials who copulate for the enjoyment of the act. There are examples of male bears performing acts of fellatio on each other, goats that perform autofellatio, otters that fornicate with dead peers, orangutans building masturbatory aids out of sticks, hyenas engaging in clitoral licking, and one case of a dolphin using an electric eel wrapped around his penis to masturbate. 
[P1] Reproduction is not the primary function of sex for humans. [P2] Other animals only have sex to reproduce. [P3] Therefore, our relationship with sex differentiates us from other species.
[Rejecting P2] Other animals also engage in sexual activities for pleasure.