If I'm reading you right, you're saying that everyone will cheat at some point so we should all be poly because there's no such thing as cheating if you're poly?
I disagree wholeheartedly.
There might be the desire to be intimate with someone else, but if you respect your partner (as Allstar has mentioned) and they were uncomfortable with something, you wouldn't do it. Polyamoury isn't just an excuse to sleep with other people is it?
In our family, if I were to meet another person who I had feelings for, and sleep with them without telling Mr.V and Ms.V first, that would be cheating, because I'd deliberately gone behind their back. Likewise, if I told them first and one of them was unhappy with the idea, for whatever reason, and I ignored their request for me not to become involved with someone else, that would be cheating too.
I don't believe people will always want to cheat, and I certainly don't think that that's a good reason to open up your relationship...
You weren't much of a muse, but then, I wasn't much of a poet...