Yes, casual sex exists. But it's always more casual for one person. I also believe it can only exist between two people for a very short term. Eventually, someone will get attached.
Casual sex can be fun, empowering, and a great way to clear sinuses. It can even be healthy. Banging it out jungle-style with someone who's only responsibility is to nibble, stroke, or suck whatever it is that needs nibbling, stroking, or sucking is liberating.
But... when it comes to two people sharing their bodily fluids, expectations are of the up most importance. I slept with this woman once. I knew going in it was casual, because she straight up told me it was. I was to be her plaything, and I happily offered my services.
The first time we did it, it was a tornado of hair, fingers, and nipples. And I left right after. Literally, caught my breath, put on my pants, and bailed. Per her instructions. The second time, I stayed a little longer. We cracked jokes. She got us glasses of orange juice. I used her bathroom to wash the sex off of me. And I left. The third time I stayed the night. I knew it was casual. She had just broken up with a boyfriend. I knew this. But I still watched her while she slept. I was the living embodiment of that that awful Aerosmith song that goes "Don't wanna close my eyes/Don't wanna fall asleep/Cause I'll miss you, and I don't want to miss a thing."
I never saw her again. She texted a few times, and I cruelly ignored them. Mainly because I knew I couldn't keep it cool. I was going to fall for her, and I pulled the ejector seat. I think it was the smart thing to do. I liked the way she smelled too much.
There is a subtle border between casual booty and the early stirrings of attachment. Find out where that is, build a tall fence, and remember which side you're on.