We evolved to enjoy sex. We are descended from thousands of generations of ancestors who really, really liked to fuck. Asking why people have sex casually is like asking why people eat food casually. Eating feels good. Food is fun. It’s often more fun when it’s meaningful and done with people we love. But that’s not always an option, and it can still be pretty darned fun when it’s grabbed on the fly with acquaintances and strangers. We evolved to want to eat food. And we evolved to want to have sex. Like, duh.