Even though the first wife’s content in a man’s second marriage isn’t necessary and, thus, you’re marriage is religiously legal, morality is one of the great elements of Islam.
It has been three years that I have had a strong affection for a married man who wanted me to death. I trust him. He hasn’t even discussed this issue with his wife yet, while his wife will never give consent to leave him. We are like lost souls without each other, we cannot bear being apart nor can we feel right with the knowledge of his wife’s discontent. Currently, the only solution he looks forward to is suicide. Is love a sin? Is divorce a sin? Or are both?
All humans have been created for a high and noble purpose. Essentially speaking, due to the fact that God, the All-Wise, is our creator, we are not created in vain, consequently we are all bound to abide by certain laws and conduct specific duties, for instance, duties pertaining to the relation between opposite genders. If one acts according to the Islamic boundaries of having relationships with the opposite gender and gets married, he can be sure that he has acted admirably according to Islamic teachings. However, engaging in a relationship outside Islamic boundaries is not only religiously unacceptable, it is bound to have social, physiological and cultural ramifications that are in some cases irreparable.
Therefore, even though the first wife’s content in a man’s second marriage isn’t necessary and, thus, you’re marriage is religiously legal, morality is one of the great elements of Islam, so please pay close attention to the following moral advice given by Imam Ali (A.S.)
His holiness writes in a letter to his son, Imam Hasan (A.S.): “My son!…Whatever you like for yourself, like for others, and whatever you dislike to happen to you, spare others from such happenings…Be kind and sympathetic to others as you certainly desire others to treat you kindly and sympathetically. If you find objectionable and loathsome habits in others, abstain from developing those traits of character in yourself. If you are satisfied or feel happy in receiving a certain kind of behavior from others, you may behave with others in exactly the same way. Do not speak about them in the same way that you do not like others to speak about you. Do not speak on a subject about which you know little or nothing, and if you at all want to speak on anything or about anyone of whom you are fully aware, then avoid scandal, libel and aspersion as you do not like yourself to be scandalized and scorned in the same manner.” 
This saying of Amir Al-Mu’mineen (A.S.) highlights this point that considering others’ desires and wishes, despite not being necessary and wajib, may have great positive impacts in one’s life as opposed to disregarding them which can result in destructive consequences both in one’s social and personal life. In every relationship with others we must initially put ourselves in their place and see whether we would like others to treat us this way? And if we like to be treated by others with a certain manner, we must also want the same for them and change our behavior accordingly. We ask you to first put yourself in the place of the wife of the man you love before doing anything else. Now would you like it if someone were to do the same thing to you?! Would you like a woman to come by one day and destroy the warm circle of your family after being married to this man, having children, establishing a family and looking forward to the future with hope and abundance? Undoubtedly, no one would accept such a thing! Therefore, we shouldn’t agree on creating this disaster for another person as well, even though your marriage may be legally permissible.
Furthermore, if this marriage upsets and troubles the first wife and her children, there is no guarantee that your ambitions and wishes will come to reality because the same thing may happen to you as well, maybe even sooner than you think; the burning love may cool down and instead of destroying one family and life you have actually destroyed two. So please keep in mind that if you sacrifice your desires for the happiness of another person’s rights God Almighty will certainly compensate in the best way and grant you a great blessing in this world or the hereafter.
- If love and affection within a relationship between two persons from the opposite gender do not fall beyond the limits set by Islam, not only will it not be haram, it is the grounds for a successful life with one another.
- Despite not being haram, divorce is known to be the most despised halal act in the eyes of Allah.
- Legally speaking, the possibility of committing suicide and feeling bad for someone cannot change the validity of a second marriage.
- In addition to the wajibat (obligations) and muharramat (forbidden acts) of Islam, there are other issues which, if neglected, can cause social and personal damages.
- Finally, what your whole question appears to imply is that you’ve proceeded in a wrong path which resulted in falling in love with married man who currently has a life with his wife. Naturally this amount of affection has not developed through a single day or with a single glance. On the other hand, the man you have feelings for has not yet been able to sort out his life and get his facts in order and perhaps may even marry you and afterwards feel horrible about his previous wife and never be pleased by his new life.
Based on the above facts, without considering your marriage haram, our advice is to carry out a righteous deed by sacrificing your desires for God’s sake and encourage the other person to ponder deeply and continue his life with his wife or to ultimately, somehow discuss the matter with the man and his wife together and come to some sort of agreement and reconciliation to join the family as his second wife without having a divorce take place.
 Mu’minun:115 (Did you suppose that We created you aimlessly, and that you will not be brought back to Us?).
 Nahjul-Balaghah, Farsi translation by Sayyid Ja’far Shahidi, letter 31, pg. 301, The Scientific and Cultural Publications Company, Tehran, fourteenth edition, 1999.