Stability is definitely always an issue no matter what the dynamics. And I think all the other things you mention about status and money tie into stability as well. It just seems like stability must always be more of an issue in monogamy though, because there is only one egg in the basket! More people means more potential support for tough times. Logistics and timing can be more complicated, and could cause instability, but I bet there are monogamous couples who have timing issues just because their lives are generally busy.

I have to wonder though whether those "reasons" you cite for cheating aren't also just rationalizations on the part of monogamous folk - are those cause, or effect? And at any rate, if people seek the company of others at unstable times, isn't that itself an argument for multiple relationships?