From the standpoint of logic it can be argued that if one person can have love for two parents equally or for two siblings equally then the same should hold for two boyfriends or girlfriends equally.
Umm, my dad was my favorite parent as we had similar interests. My older brother was my favorite for the same reason, not to mention my younger brother was a massive tattle tale.
This compersion is similar to the joy that parents feel when their children get married
Umm, you mean the evil parents that ridicule their child's partner for every and anything? I'm sure you know the stereotypes about mother in laws. If you don't, then you need to watch some movies and/or tv shows. There are countless ones to choose from.
as such restrictions can be used to replace trust with a framework of ownership and control.
You said that about monogamy, but I'd say it's the complete opposite.
Most of the couples I know that have "open" relationships, one of the partners only grudgingly agreed to it.
One partner wanted to whore, while the other wanted a monogamous relationship, but only gave in to make their slut of a partner happy while making themselves miserable.