Jesus was conflating two different ideas, the first was that of the "two flesh becoming one", and the second was "what god has joined together let no man put asunder", neither of which occurs anywhere in the Old Testament except for the two becoming one flesh
in Genesis 2:24. There is no way a man can be of one flesh with multiple wives, so this concept of monogamy that Jesus was introducing was something that the pharisees would have been unfamiliar with as a biblical teaching. Also, as I mentioned in my previous post concerning divorce, God himself was the one who gave Moses the laws concerning divorce...it wasn't something Moses came up with himself.
Have a good night,