It isnt about what you do, it's about how you make the other person feel.
If you flirt with others all the time and that upsets them, thats just as bad as doing something. It's not good to hurt people.
You can cheat emotionally. (thats not to be confused with having a crush on someone else while in a relationship. Thats one thing. Telling your crush you fancy them and being soppy together and huggy but not kissing is another)
There's a chance I'd forgive a kiss,if they were drunk, or it was brief, if I really liked the person, but if they put me through psychological torture by always flirting with others, and having special soppy relationships with friends they admitted to fancying, I'd be more likely to end things.