Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
In fact, you have made a mistake by marrying a second wife against the will of your first wife and your parents as well.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
If you have married a second wife without the permission of your first wife and against the wishes of your parents, you may divorce her. This is especially the case if you married your first wife in a milieu where monogamy is the norm.
In this case, your first wife agreed to marry you with the tacit understanding that she would remain the sole wife, and she would not have agreed to marry you if you had disclosed to her the intention to marry a second one.
Therefore, if you did marry a second wife without her consent, then you are guilty of violating the terms of your marriage contract. The prophet (peace and blessings be upon him) says, “Muslims are bound by the terms of their contracts and even more so the terms of their marriage contracts.”(At-Tirmidhi)
Allah Almighty knows best.