I've cheated once before, and whilst I don't think it was particularly honest or nice of me, I don't regret it anymore. I felt the relationship was going nowhere, my partner would spend a tiny portion of their time actually talking to me (it was long distance, talk was all we had) and when I tried to voice my dissatisfaction they blanked me entirely - when my year long crush walked into my life, I went for the chance and told them the first chance I got.
A year later, I became openly polyamorous with my new partners blessing. In december it'll be our 4 year anniversary. As far as cheating goes, well, I've been quite open with every partner I've had that I don't mind if they form relationships with others as well as me. I'm not the jealous type, and I trust them to keep the number of attachments they form to a number they're comfortable with. That all said, I think that to lie and betray someones trust is a pretty big deal, and relationships do come with assumed monoamory. I don't like cheating - but I'm fine with people being open.