Sexting with other people while in a relationship.

Depends upon how and why it is done. 1. Is it done behind partner's back or with their knowledge? Secrecy for whatever reason makes it cheating, or you wouldn't be keeping it secret. 2. Is it with one person, or multiple? Doesn't have so much to do with the cheating issue, but a lot to do with perceived threat and prodding insecurities. 3. What kind of sexting and why? If it is just fantasy about things your partner is not interested in discussing or doing, what's the harm?

For the record: I used to do a lot of cybering in the old chat rooms, before texting was even a thing. My wife knew about it because I told her. She wasn't thrilled by it but wrote it off as male midlife crisis, knowing it was fantasies she didn't care to discuss or do. For my part, I made it clear with every partner that I was happily married and our cybering was never going to lead to anything in rl. Fact is, my wife and I actually would up meeting a half-dozen of the women solely for platonic friendship because I found them interesting as did she. She only had issues once when the woman came to our meeting with a clear objective beyond the groundrules I had set about nothing happening in rl