People often want to make polyamory as close to monogamy as possible in some attempt to correct all the things that are morally wrong about it.
This is where I'm starting. People who fundamentally believe that non monogamy is wrong so they structure their relationships in a way that reflects monogamy. Everything else follows.
I'm saying that there are a subset of people who practice non monogamy who actually believe it is wrong and feel guilty/dirty/wrong for desiring and partaking in it.