Contrary to popular belief, a woman may stipulate in the marriage contract that she be the only wife. If he marries another, then she has the right to seek khula. No one is forcing the brother to accept that condition. If he accepts it than he MUST abide by it or she has right to leave the marriage. Please read below.
Ibn Qudaamah (رحمه الله) said:
“If he stipulates that he will not take her out of her house or her city, or that he will not travel with her or will not take another wife, then he is obliged to fulfil that, and if he does not do so, then she has the right to annul the marriage. This was narrated from ‘Umar, Sa’d ibn Abi Waqqaas and ‘Amr ibn al-‘Aas (رضي الله عنه). “
Shaykh Ibn ‘Uthaymeen (رحمه الله) said:
The basic principle with regard to conditions in the marriage contract is that they are valid, unless there is proof to show that they are not valid. The evidence for that is the general meaning of the evidence which speaks of fulfilling covenants:
“O you who believe! Fulfil (your) obligations”
“And fulfil (every) covenant. Verily, the covenant will be questioned about”
and in the hadeeth narrated from the Messenger (ﷺ) it says:
“The Muslims are bound by their conditions, except a condition that forbids what is permissible or permits what is forbidden.” (Tirmidhi).
And he (ﷺ) said:
“Whoever stipulates a condition that is not in the Book of Allaah it is not valid, even if he stipulates a hundred times.” (Bukhaari and Muslim)
To sum up, the basic principle with regard to conditions is that they are permissible and valid, whether they are to do with marriage, buying and selling, renting, pledges or mortgages, or awqaaf. The ruling on the conditions that are stipulated in contracts, if they are valid, is that they must be fulfilled, because of the general meaning of the verse (interpretation of the meaning):
“O you who believe! Fulfil (your) obligations” [al-Maa’idah 5:1].
Al-Sharh al-Mumti’, 5/241 (Egyptian edition).
Shaykh Ibn ‘Uthaymeen (رحمه الله) said:
“If she stipulates that he should not take another wife, this is permissible. Some of the scholars said that it is not permissible, because it is restricting the husband in something that Allah has permitted to him, and it is contrary to the Qur’aan in which it says (interpretation of the meaning): “then marry (other) women of your choice, two or three, or four” [al-Nisa’ 4:3]. It may be said in response to that that she has a reason to ask him not to marry another wife and she is not transgressing against anyone. The husband himself is the one who is giving up his right; if he has the right to marry more than one, he is giving it up. So what is to prevent this condition being valid?
Hence the correct view with regard to this matter is the view of Imam Ahmad (رحمه الله), which is that this condition is valid.” Al-Sharh al-Mumti’, 5/243
Shaykh Saalih al-Fawzaan (may Allah preserve him) said:
“Among other conditions that are valid in marriage is if she stipulates that he should not take another wife. If he fulfils the condition (all well and good), otherwise she has the right to annul the marriage because of the hadeeth, “The condition which most deserves to be fulfilled is that by means of which intimacy becomes permissible for you.” Similarly, if she stipulates that he should not separate her from her children or parents, this condition is valid and if he breaks it, she has the right to annul the marriage. If she stipulates that her mahr should be increased or that it should be in a specific currency, the condition is valid and binding, and he has to fulfil it, and she has the right of annulment if it is broken. In that case she has the choice and may decide any time she wants and may annul it whenever she wants, so long as there is nothing on her part to indicate that she accepts it if she knows that he has gone against what was stipulated; in that case she would no longer have the option.
‘Umar ibn al-Khattaab (رضي الله عنه) said to the one who he ruled was obliged to fulfil what his wife had stipulated, when the man said, “Divorce us in that case,” ‘Umar said: It is a must to fulfil the conditions, because of the hadeeth, “The believers are bound by their conditions.”
Al-‘Allaamah Ibn al-Qayyim (رحمه الله) said: It is obligatory to fulfil these conditions which are the most deserving of being fulfilled. This is what is implied by Sharee’ah, reason and sound analogy, if the woman did not agree to become a man’s wife except on these conditions, and if it were not obligatory to fulfil them, then the marriage contract would not be based on mutual agreement, and it would be making something obligatory upon her that Allah and His Messenger have not made obligatory.”Al-Mulakhkhas al-Fiqhi (2/345, 346)