muchymozzarella:

Cheating isn’t bad because you had sex with another person, cheating is bad because you took away your partner’s choice while simultaneously showing you don’t trust them the way you would trust a life partner

The worst two things you can do in a relationship is make a decision that affects you both without once consulting your other half, and breaking trust. And you do both when you cheat. The having sex part isn’t even the issue; a lot of couples assent to relations with others. It’s about being honest with your partner about your wants and needs and negotiating with them how to handle it in a way that’ll be good for you both.

No honesty and no choice = no relationship