I personally feel that cheating is having sex then NOT discussing it with your partner. My exwife used to meet guys at parties and if it was leading to sex she would find me and tell me to follow her to a bedroom if it was a group of guys, if not she would do a one on one and tell me later. This progressed to her going out with a group of good looking friends and usually pulling by the end of the night. She would then tell me all about it after she got home and we were having sex, usually with other men's cum in her pussy.
After a while the sex talk got less and less until nothing, even though she still went out on Friday night with her friends. One her friends once told me about some of my wife's meets, but it only confirmed what I knew. I eventually walked in through the back door and caught both her and her cousin fully naked with him balls deep and cumming inside her. The affair had been going on for months apparently. For me that was enough.
Even though her sex was amazing, her deception was her real kink