Well this is a big old can of worms! I have never cheated on a partner, but I have been cheated on. It left me feeling divested, sore and pretty much unable to trust people. I think if you do anything from flirting to sex , without your primary partner know and being ok with it, is truly a vile thing to do to someone you are meant to be in love with.
I have an open , poly , none monagmse realstionship. One of the rules we set down at the start of it was that we tell everyone we are involved with , what we are doing and who we are doing it with. I personally don’t play with or fuck other anyone without the boss man being with me. but the girls do see others away from are family , but we know what they are getting and its all fine.
The thing that I really dislike , is the people who start flirting or start affairs with people who already have a partner , just because they can. Knowing full well the upset and hurt that it e=will cause when they get found out. The is no need for that type of spitefulness in a world that is already full of pain and suffering already!