People have sex for many different reasons. The more obvious reasons are pleasure and closeness (and for a few people, conceiving). The covert reasons (which a person may or may not be aware of) include feeling attractive; feeling powerful; feeling normal; wanting something in return; and getting revenge on a third party.
This is all pretty standard stuff.
But for many people these days, sex seems to be about something else entirely—not so much about sex as an activity, but as an idea. An identity. A statement of self, of tribe, of belonging. Maybe as a focus of civil rights activism.