Agreed. I don't think divorce or break-ups happen any more or less in polyamorous relationships than they do in mono ones. The most common reasons might change a bit, i.e. perhaps slightly fewer break-ups so that someone can date someone new...
It's worth noting that your ex and the other guy probably didn't identify as polyamorous. You described it as a swinging relationship, which generally means you're only romantically involved with one partner. For whatever reasons, she preferred to be romantic with him. Probably a "grass is greener" situation that will eventually crumble, but that's neither here nor there.
Actually, I've never understood the rationale behind urging someone to leave their partner to be with you. How is it that they don't realize: someone who did that once is capable of doing it again and again, and it's only a matter of time before the tables turn?
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."