Here’s a Bible passage you can use with the next Christianist who tells you that marriage was ordained by God to be one man, one woman from the beginning of time (bolding mine):
That night, however, Jacob arose, took his two wives, with the two maidservants and his eleven children, and crossed the ford of the Jabbok.24After he got them and brought them across the wadi and brought over what belonged to him,25Jacob was left there alone. Then a man* wrestled with him until the break of dawn.26When the man saw that he could not prevail over him, he struck Jacob’s hip at its socket, so that Jacob’s socket was dislocated as he wrestled with him.d27The man then said, “Let me go, for it is daybreak.” But Jacob said, “I will not let you go until you bless me.”28“What is your name?” the man asked. He answered, “Jacob.”e29Then the man said, “You shall no longer be named Jacob, but Israel,* because you have contended with divine and human beings and have prevailed.”30Jacob then asked him, “Please tell me your name.” He answered, “Why do you ask for my name?” With that, he blessed him.31Jacob named the place Peniel,* “because I have seen God face to face,” he said, “yet my life has been spared.”f
Jacob not only had two wives, but God had no problem with it and blessed him. If marriage was really one man, one woman, since the beginning of time, God would have have blessed Jacob. At least not without insisting that Jacob dropped his second wife.