Actually, DW, that isn't the case about humans being the only ones who participate in recreational sex. Once the folks who study these sorts of things started looking at all behavior, and not just looking for certain types of behavior, especially in primates but by no means exclusive to them, they found a lot of recreational sex. And a lot of same-gender sex. A book came out a few years ago dealing with this topic; I can't locate the title right now.

SRS