It’s a completely different beast- as different as “having a mistress” is to “taking a high school sweetheart to the prom.” With a girlfriend, you have an expectation of monogamy. You probably have a formal agreement. It’s wrong to break that. Without being “boyfriend-girlfriend,” in a relationship, or otherwise having negotiated exclusivity, you don’t have an expectations of monogamy, you can’t “cheat,” and it’s not immoral to have sex with whoever.
Hooking up does happen, but it’s different from peer group to peer group. I’ve been in some situations where people hook up like crazy- I remember eating dinner at a table where three girls and a guy who had all at some point hooked up with the guy, and everyone was teasing him about it- as a totally casual, not-at-all emotional thing. I’ve had other peer groups where it basically never happens.