Casual dating is often (but not always) nonexclusive. People commonly assume its fine to see other people unless theres been an explicit discussion about exclusivity. Still, its always wise to have a convo about exclusivity at some point just to make sure everyones on the same page.

Is it okay to cheat in a casual relationship?

If youre truly casual, its not an emotion-based relationship. And if its a real relationahip - where you are a couple sharing your lives together - then it isnt casual. Cheating is not infedelity in the strictest sense, but you will hurt your partner, which is bad.