All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If a man knows that a given woman does not accept to marry a married man, then it is not lawful for him to pretend to her that he is not married to deceive her. No doubt, it is cheating and deception. Also, it may cause many problems in the future and will probably lead to separation. However, if you do not know that she will refuse to marry a married man, then, in this case, you are not obliged to inform her. Yet, it is better to inform her since fairness and clearness from the very beginning help to have a happy and stable married life in the future. Therefore, we support your opinion because it is a basic element to have a good life.
Allah knows best.