This is a very interesting thread. I wish there were more responses. I really think it is a very odd topic. Where does sex begin? I've known folks who actually believe sucking cock is not sex. Anal penetration by a penis is not sex. Handjobs are not sex. Giving oneself a handjob is not sex. Etc.
Then there are those who seem to think these behaviors are sex, but they don't constitute a breaking of commitment.
I used to go to erotic massage places and get handjobs. I did this when I was in "committed monogamous relationships." I think part of my rationale was that it wasn't sex. How convenient! Really, it doesn't matter what label I put on a behavior. It's about open, honest, ethical communication.
Lots of trouble with one of my sig others started when she "caught" me masturbating. I thought she had left the house and I was getting off and she heard me. (I can be kind of loud). For her, it was real betrayal. For me, it was a source of deep resentment, a feeling I had no privacy in my own home. These sorts of collisions are the stuff of relationship catastrophe. She gets to feel betrayed by my autoerotic sex life and I get to feel pissed off at her for violating my privacy. Ugh.
If I look at porn and get turned on, is that betrayal? If I fuck someone else but think about you the entire time, dear, is that commitment? :-)