Incorrect. Really bad interpretation. Becoming one flesh has absolutely not a thing to do with monogamy.People are to become one flesh, not one dozen.
Becoming 'one flesh' is what happens when a man and woman have intercourse by marriage/union. Becoming 'one flesh' is what happens when a married person has sex with someone under the definition of 'adultery'. Becoming 'one flesh' happens when two people unmarried have intercourse.
Becoming 'one flesh' describes the intimate act of a male and female in intercourse.
David became 'one flesh' with each new wife he married prior to and after he became a king. Abraham became 'one flesh' with Sarah's handmaid.