I think a good rule, when it comes to cheating (and I'm sorry; I realize this is off-topic, but it's brief), is what I've heard numerous times: If you're afraid to tell your partner what you're doing, it's probably cheating.

Even if you don't consider it cheating, but you know your partner would be hurt or angry, you shouldn't do it if you want to remain with your partner, rather than do it behind his/her back. If it's more important to you to do whatever you're doing, then don't be in the relationship.