DOMINUS said"Cheating" is how the parties in the relationship define it.
That's what I think. I know couples who have varying degrees of an open relationship, but they don't consider that cheating. Others (like us) are strictly 100% monogamous, and any kind of sexual fooling around outside the relationship would be serious cheating.
And it doesn't have to be cheating in the strictest sense. My first, late partner didn't want me to have porn or any beefcake pics on my computer, or to visit those sites. I thought it was just innocent eye candy, but it apparently made him feel insecure, like I felt he wasn't adequate for me.
Well, a bird in hand is worth 2 on the screen. And so I wasn't gonna jeapordize what I had by upsetting him, with something that was far less important to me than he was. A small sacrifice, and he had no problem with me continuing to visit online gay social sites, as he did.
We had lots of friends there, some of whom visited us and stayed at our place. Neither of us feared the other was doing clandestine cruising. But my looking at male nudes & porn bothered him, so I stopped.