I wonder if D understands why he feels the needs monogamy. I'm not saying that it's wrong of him to want that by any means, but it would be interesting to know whether he's really explored the possibilities or whether it's an issue of fear or jealousy or societal expectations that could be worked out.
Great essay on why people are not interchangeable, and why it hurts to lose a relationship even if you have more than one: http://www.xeromag.com/fvessay06.html
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.