"And I would be fine if my partner saw someone else too." So why cheat then and not talk to your partner openly and come to an agreement? I know plenty of coup

"And I would be fine if my partner saw someone else too." So why cheat then and not talk to your partner openly and come to an agreement? I know plenty of couples that let each other to have other partner(s), and that's totally normal. Playing with another human being feelings, on the other hand, is not imho.