The heteros have even more destructively "redefined" sex itself: obliterating its natural connection to procreation via drugs, devices and surgery. This is every bit as radical as obliterating one's natural gender by drugs, devices and surgery.
Once a society has accepted divorce and contraception, there isn't much "definition of marriage" left: it's been pretty thoroughly smashed. The gays and the poly's are just scavengers sifting through the rubble and picking out the garbled fragments they fancy.
EXACTLY