Dear Sister Salam.
4:19 O You who have chosen to be graced with belief! It is not lawful for you to force women into marrying or holding on to them in marriage against their will. Pressuring women to remain in wedlock by threatening to take away the marital gift is forbidden. A wife could forfeit the right to the marital gift only if she has indulged in clear lewdness. You shall treat your wives nicely. Even if you dislike them, it may happen that God has placed much good in what you have failed to realize.
According to 4:19 it is unlawful to keep a woman in wedlock under compulsion (كَرْهًا) therefore it is a duty of a husband to remain at a distance from those actions giving feelings of compulsion to a wife. In my humble opinion a wife can accept second wife only under compulsion (e.g. pressure from family , economic pressure , pressure of society etc ) and because of this reason the moment a husband takes second wife , his first wife becomes unlawful for him . Again in my humble opinion those who take second wife and use 4:3 as justification are in gross violation of Qur’an as 4:3 deals with an extraordinary situation and even in that situation ask a believer to give priority to the rule “THEN ONLY ONE”.
Having established this principle , Qur’an says
4:20 If you wish to marry another wife in place of your present wife, and you have given her a great deal, you shall not take back anything. Would you take it back by way of slander, a transgression that will drag down your own ‘self’?
In place of (اسْتِبْدَالَ) in 4:20 is a clear indication of the Qur’anic view that a monogamous marriage is the desirable norm.