While some polygamists may feel they were “born this way,” their basic sexual orientation is accommodated under existing marriage law even if the breadth of their affections isn’t, which makes them less sympathetic than same-sex couples even if their legal arguments sound similar.
The reason plurals doesn't flourish, to you, is because the people who practice it don't tell you, lest you tell them what you just told us, and the inevitable "are you Mormon?"
Men and women have been found by psychologists in every culture and tradition to differ in aggression and general activity level, types of cognitive strength, and sensory sensitivity. The differences between man and woman are obvious to all but the most ideologically blinded deconstructionists.
"Nothing about marriage has anything to do with liberation or equality, except in these modern times"?
You're going to freak out when you learn about the divorce rate.