Well, I hope to do something more detailed in the near future, but here's my thoughts on it, off the cuff. My opinion on default monogamy isn't a criticism of the results (being with just one person), it's a criticism of the process (not making a conscious decision to be with just one person). Anytime people make a major personal decision -- and how you choose to lead your life with your significant other(s) definitely qualifies! -- they should decide not because of societal norms but because of who they are, whom they are with, and what they hope to make of their life.
Now, do I think that personal stagnation results directly from default monogamy? No. But I guarantee you that those who choose monogamy -- or any other relationship style -- are better communicators and more in tune with the needs of their partner(s).
Hope that helps. Sometimes I ramble...