I think most animals mate when the female is in season. The males can smell it and if she's not in season she doesn't want anything to do with the male and will fight him off. So basically it is to procreate and maintain the species.

Maybe it is the males that get "horny" and hump anything in sight. I grew up around dogs, cats and horses so those are the only ones I've observed. I haven't seen a "gay" female animal.