You missed one acceptable scenario where you can biblically be one flesh with more than one person. The death of a spouse and marriage to another.
Did Jesus condemn the Samaritan women at the well who had five husbands and the man she lived with was not her husband. No, Jesus offered her the water of life that leads to eternal life. Why do we pretend that politicians are morally upright people? Hillary is just as immoral as Trump. We had a choice between two badly flawed people. I voted for Trump policies over Hillary’s policies and I believe fewer babies will be slaughtered in the womb and their body parts offered on the auction block to the highest bidder. Maybe we should take a break from Trump’s depravity and look at the depravity of a nation that allows babies to be slaughtered in the womb and the parts sold on the auction block, all in the name of choice. This is a kin to the sacrifices made in the high places, on the alter of selfishness.