It was in the summer of 1873 that 'Abdu'l-Baha [then 29 years old] was married to Munirih Khanum, the niece of two brothers who were devoted followers of Baha'u'llah in Isfahan [six years later they became King and Beloved of the martyrs] She had been brought to 'Akka by Baha'u'llah in early 1873, at a time when the situation was very difficult due to the murder of the Azalis and all other Baha'i pilgrims had been told to stay away. 'Abdu'l-Baha himself had been reluctant to marry, although a number of spouses had been proposed to him over the years. He felt he could serve his father better if he remained single, as indeed did his sister Bahiyyih Khanum who never married. Although he had turned down the other spouses who had been proposed to him, he was attracted to Munirih Khanum and agreed to marry her. The marriage was put off for a few months however because the tight accommodation in their place of residence meant that the couple could not have a room to themselves. llyas 'Abbud, their neighbor who lived in the other half of the same building and who had been so fearful of the Baha’is earlier, had by this time been won over by 'Abdu'l-Baha. Bahiyyih Khanum [‘Abdu’l-Baha’s sister] went to the wife of Ilyas 'Abbud and told her of the problem. When he was informed, 'Abbud immediately opened up a doorway between his residence and Baha'u'llah's and gave ‘Abdu'l-Baha a room in his house for his use. Once this happened the marriage was able to proceed.
Four daughters were the surviving children from this marriage; the two sons who were born both died at about the age of two and three other daughters died in infancy. In later years, 'Abdu'l-Baha was put under a great deal of pressure to marry again so that he would have a son to succeed him, but he declined to do this. Bahiyyih Khanum explained many years later:
“Many influences, and those of the very strongest character, have been brought to induce my brother to take a second wife – a practice which the Blessed Perfection [Baha'u'llah] did not in terms forbid, but advised against. The believers have urged it strongly for several reasons. Very many of them wish to take a second wife themselves, but feel constrained from doing so by the Master's ['Abdu'l-Baha's] example. In Persia, except among believers, polygamy is a universal custom, and the restriction to one wife, which all believers feel and respect, seems very severe. Then there is a general wish that the Master might have a son to succeed him. Other arguments have been advanced; and the pressure brought to bear upon him has been, and still is, very great -- greater than you can easily imagine.
The general advice of the Blessed Perfection against a second marriage would in itself have had the effect with my brother of a command and have settled the question; but as regards him it was withdrawn by our Lord before his death. He said to ‘Abbas Effendi ['Abdu'l-Baha] that He rather wished to lead the believers gradually to monogamy than to force them to adopt it, which they felt hound to do by reason of the Master's example; that therefore, and since it was much desired by all that the Master should have a son, He withdrew even the advice in his case, and desired him to consider himself free to follow his own desires and inclination.
To this the Master replied that his own wishes and feelings were against a second marriage, though, if the Blessed Perfection should command it, he would obey. This, however, the Blessed Perfection never did.
To all other appeals his reply has always been a firm refusal. He thinks that if it had been God's will that he should leave a son, the two who had been born to him would not have been taken away. He believes that the best and highest condition of life for a man is marriage to one wife, and that it is his duty to set that example to the world.” ['Abdu'l-Baha later carried out his Father's wish to make monogamy a binding law on the Baha'is] (Quoted in Phelps The Master in ‘Akka, pp.120-1) (Moojan Momen, ‘Baha’u’llah A Short Biography')