Making a condition that husband cannot take a second wife while married to the first wife

I once asked my ustaaz whether it is okay for a woman to state in the nikaah contract that while she is married to the man he cannot get another wife. I was told that it is incorrect to do such a thing because it is permissible to have more than one wife and you are not allowed to make it impermissible upon him. Basically you can't make haraam upon him that which is halaal. Could this please be clarified.