I can't speak about anyone else but the root of my monogamy is my natural inclination to focus all romantic loving energy into one person.
When I think of monogamous relationships, such as the ones I have had, the desire to have that singular focus returned to me had nothing to do with jealousy or insecurity - it was rooted in wanting to have love communicated to me in the same manner as I was communicating it to them. Sharing their heart or body with another simply wasn't a part of that.
The concept of insecurity and jealousy being at the base of monogamy totally makes sense to me in a poly atmosphere however. It vilifies the behavior that prevents a non-monogamous partner from achieving the freedom they want. Inversely, a monogamous partner who vilifies the poly desires of their partner as promiscuous or non-committal also makes sense in a monogamous atmosphere.
Not all people will feel negative about the different approaches to love their partner has. There is no blanket that covers people as a group - this issue is individual based.
The concept of jealousy and insecurity for monogamy has as much validity as the concept of being over sexed and non-committal forming the base of polyamory......Neither are wrong 100% of the time but both can be very right.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over