You cannot always say, I don’t know where it’s written…
YEs you can, and you should. People have the absolute right to know the HALACHIC (not minhag of some, not chumros of some)sources when someone is trying to tell them they are doing something the wrong way. Otherwise, the person who is giving them mussar should stay silent.
BTW, not only does it NOT say in the Torah that a man cannot have more than one wife – The Torah outlines a whole series of halachos exclusively devoted to the proper way to treat the additional wife and her children. It was Rabbeinu Gershom who made the takana to have only one wife (and not to open other people’s mail), and it was for a self-limiting time period, I believe, (and that time has passed, but we nonethless have not reverted back to legally bigamous marriages). Hashem allowed MANY things that we no longer do today because of the takanos of our Gedolim, who were invested with the halachic power to do so. But if we accept that the Torah is perfect as it came from Hashem, it does leave way to say that His intention was a little different from theirs, but they obviously saw a need to enact gedarim to preserve His intent.