Easy: being on top of the food chain allowed us to evolve our relations beyond having an offspring

Those having more satisfactory relations formed groups that we now call families up to the point where sex today is mostly recreational

basically pleasurable sex gave us mate choice just like a bird chose the best dancer or a lioness the stronger male

pleasure instead of strength in the long run made happiness a goal in our modern societies instead of simply domination

we don't have sex just in some period of the year

we enjoy sex just like we enjoy eating or having fun

For a cat or a dog sex is not pleasurable, it's just a relief from heat, but we call it rape.