That is NOT condoning cheating though, which is never okay. Huge difference between doing things to yourself to relieve stress and climbing into bed and destroying trust.
In a relationship, you're still going to find other people attractive.
That's fine, natural and normal. It's not uncommon for people in relationships to develop little crushes with other people.
Well your attraction to other people doesn't fade away when you're in relationship. However if you are someone who prefers not be tied down to one person for fear you can't control yourself, then a monogamous relationship isn't for you. A more suitable relationship might be an open relationship, where you and your partner agree on whatever loose boundaries you have.
I'm a monogamous kind of person and I found a whole lot of people attractive while I was dating someone.