Thank you for your post - I was trying to think my way through what I would answer the OP, and then read your post - you did it far better than I could have, and far more thoroughly.
Poly folks tend to be ones that are willing to question the societal judgements or terminology (like "cheating"), morality, and ethics. They don't accept things simply because "society says so", or is it written in some religious tome. If you brother is always going to refer to the societal norms, then you aren't going to get anywhere.
On the commitment aspect - a counselor friend of mine who became interested in polyamory from a professional standpoint, told me that if poly is done well by those that want commitment, there is a lot MORE commitment present than in most monogamous relationships. If your brother were to meet some poly relationships and could talk to them, he would see that.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb
Last edited by CielDuMatin; 06-25-2012 at 05:27 PM.
Reason: Adding comment about committment