In most cultures betraying contracts is usually considered bad, and relationships are basically intimate and emotional contracts. Trust is important even in business contracts, but in interpersonal ones its ESSENTIAL. If you entered into a polyamorous relationship, cool, thats the contract all parties agreed to, but if it wasnt specifically polyamorous its defaults to monogamous, and if you breach that trust, you’re probably not getting it back, and without trust ever having a healthy relationship again is probably gone