Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The enquiring sister should be aware of that Allah has permitted a man to marry more than one wife provided he is able to fulfill the material and immaterial rights of his wives. No one can change or review this ruling: Allah Says (interpretation of meaning): {Should not He Who has created know? And He is the Most Kind and Courteous (to His slaves) All-Aware (of everything).}[67:14]. The clear verse that proves this permissibility is the following verse: {…then marry (other) women of your choice, two or three, or four but if you fear that you shall not be able to deal justly (with them), then only one or (the captives and the slaves) that your right hands possess….}[4:3]. Whoever believes in Allah and in the Day of Judgement should subject to this ruling without having any objection. Allah Says (interpretation of meaning): {It is not for a believer, man or woman, when Allâh and His Messenger have decreed a matter that they should have any option in their decision. And whoever disobeys Allâh and His Messenger, he has indeed strayed in a plain error.}[33:36].
As for the agreement you had made with your husband at the time of marriage contract that he should not marry another woman, Muslim scholars have two different opinions concerning this matter.
First, the second marriage is correct. The condition, which is made at the time of marriage, is null thus the husband is not bound by such a condition. This is the opinion of Imam al-Shafi'e and others. They present the following proofs to support their opinion.
1) The Prophet (Salla Allahu Alaihi Wa Sallam) said, Muslims must keep to the conditions they have made, except for a condition which makes unlawful something which is lawful, or makes lawful something which is unlawful.
2) Another Hadith reads, What about the people who stipulate conditions which are not present in Allah's laws? Whoever imposes conditions, which are not present in Allah's laws, then those conditions will be invalid, even if he imposed these conditions a hundred times. Allah's conditions (laws) are the truth and are more solid.
3) There is a Hadith that the Prophet (Salla Allahu Alaihi Wa Sallam) said, Whoever stipulates such conditions as are not in Allah's laws, then those conditions are invalid even if he stipulated a hundred such conditions.
Imam al-Baihaqi reported from Abu Hurairah that the Prophet (Sallallahu Alaihi wa Sallam) said: Muslims are bound by their condition which are in accordance with the truth. It is also reported in Sunan al-Baihaqi that a man in the period of Umar (Radiya Allahu Anhu) married a woman and stipulated a condition that he will never take her out of her house. When the case came to Umar he cancelled the condition and said: 'a wife is always with her husband'. Imam al-Baihaqi said There is another narration that contradicts this one. But this narration is more likely according to the proofs of Qur'an, Sunnah, and the sayings of the companions of the Prophet (Sallallahu Alaihi wa Sallam). They say that the condition is null because it makes Haram what is Halal, i.e. a second marriage. In addition, such a condition is not related to the purpose and the interest of the marriage contract.
Second, the marriage is valid and the condition is also valid. This is the opinion of many companions of the Prophet Muhammad (Sallallahu Alaihi wa Sallam) and others. This is also the opinion of al-Awzai, al-Hanabilah, al-Malikiyah. They cited the following evidences to support to their stand.
1) Allah Says (interpretation of meaning): {O you who believe! Fulfill (your) obligations….}[5:1].
2) The Prophet (Sallallahu Alaihi wa Sallam) said: Muslims must keep to the conditions they have made. [Al Bukhari]
3) It is also reported that the Prophet (Salla Allahu Alaihi Wa Sallam) said, From among all the conditions which you have to fulfill, the conditions which make it legal for you to have sexual relations (i.e. the marriage contract) have the greatest right to be fulfilled.
4) Al-thram reported that a man married a woman and stipulated the condition that he will not move her from her house; later on he wanted to take her with him. They brought their dispute in front of Umar who said, "She has the right to act according to by her conditions."
5) In addition such a condition is in favour of a wife and she gains benefits from that condition, and it does not contradict the purpose of the marriage. Therefore, he has to fulfill that condition as he fulfills the condition of increasing her Mahr if he had made the condition. Ibn Qudamah preferred this opinion saying that this condition does not contradict Shariah. Therefore, it is a valid condition and one should fulfill it.
Those who hold the latter opinion also justify their opinion that such a condition does not make lawful what is prohibited since a woman gains the right of canceling the marriage contract by this condition if the husband does not fulfill it.
Therefore, you have the right of canceling the marriage. But our advice is to be patient and be content with what Allah has decreed if your husband does not fulfill his condition. You should not think about committing any sin such as killing yourself or disobeying your husband which may expose you to the curse of angels. At the same time we advise the husband to fulfill his conditions. However, if he actually feels temptation, has fear of committing Zina due to his powerful sexual desire, can bear the expenses of the second marriage, and he is able to deal justly among his wives then we suggest that he marry a second wife. In practice many women tolerate and allow their husbands to commit Zina but they do not want to share a woman as a co-wife. This practice is totally against what Allah has instructed for us.
Allah knows best.