Ever seen Startrek Enterprise? Like with Dr. Flock's species, can't remember the name?
Only if it is consensual and everybody is aware I guess. Not my cup of tea.
Because of family. No one likes people of either gender who just sleep around. They arent dependable and arent mother/father material. There are other reasons for it too. No one really likes to share their partners. Or if they dont mind, the relationship isnt that serious or is one sided. Most men who are ok with polygamy if they can have multiple wives suddenly have a problem if their wives also have multiple husbands.
And if this really was a thing ad could work, how would you imagine a family them? A whole country? Your 5 wives have 5 husbands each who also have 5 wives each and so on.. each of them with at least 1-2 children..
Polyamoury just means more than ONE relationship, not 5. Even so, if I did have 5, why do you care?
Don't insult my chicken sandwich, simply because you like BLTs.