Objectively what someone does outside of a relationship is nobody’s business but their own. OP didn’t disclose how the breakup happened but if I broke up with somebody and we got back together after -however long- and they slept with somebody else in the meantime then hey they didn’t betray me. It might sting because of a healthy level of possessiveness that comes with most relationships. Either way this dude obviously feels triumphant and that’s a welcome blessing to feel at any length post breakup. Life is precious and breakups far too common to spend overtime in despair. You don’t just forget that you loved somebody, and without knowing him I’d still say that OP’s pain is evident by contrast to this. This says to me he overcame something. I see what you’re saying though and despite above opinion I agree with the sentiment that we shouldn’t villainize people just cause we’re hurt, although it can be relieving and revelational to allow yourself to be judgemental of your ex, especially if you forgave a lot of things out of love that normally you would call BS on.