"No, cheating is never okay in any type of relationship, even if you're just dating casually. The only time you should have relations with someone other than your partner is if you talk to your partner and you both agree that having sex with other people is acceptable. This isn't a complicated concept."
How can you "cheat" if you're just casually dating? You can only cheat on someone in a relationship where both of you have agreed to remain exlusive to one another and not see other people/have sexual relations with other people. Up until the point where both parties have that discussion, it really seems like fair game.
It makes absolutely no sense to me why I should be expected to remain exclusive to someone if I've only been on two or three casual dates with them, like coffee or the movies and such. If you don't want the person you're seeing to be banging other people, it's your responsibility to speak up about what you want and lay out ground rules.