Q: we knew about Muhammad's marriages, he had 9 wife's.
What's the reason?
A: All-Praise is due to Allah,
With regard to the question about plural marriage in Islam and its being forbidden in the New Testament, it should be noted that Allaah gave each Messenger his own laws and path. Allaah enjoined Tawheed (belief in the Oneness of Allaah) upon every single Prophet whom He sent, but the laws varied and some of them abrogated others. Some things that were permissible at the time of Adam (peace be upon him) were abrogated at the time of Nooh (peace be upon him).
The laws that existed at the time of Moosa (Moses – peace be upon him) were partially abrogated at the time of ‘Eesa (Jesus – peace be upon him), as Allaah tells us (interpretation of the meaning):
“To each among you, We have prescribed a law and a clear way”
So once this is understood, we will realize that plural marriage did not exist only in the law of Muhammad (peace and blessings of Allaah be upon him), rather it existed in the laws of all the previous Prophets. For example, Ya’qoob (Jabob – peace be upon him) had two wives and was married to two sisters at the same time, according to the Old Testament, as it says in the Book of Genesis 29:15-35.
The father of the Prophets, Ibraaheem (Abraham – peace be upon him) was also married to two women, Haajar (Hagar) and Saara (Sarah). The Old Testament mentions that the Prophet of Allaah Dawood (David) had seventy or ninety-nine wives, and Sulaymaan (Solomon) had one hundred wives. These and other examples demonstrate that each of the Prophets implemented the laws that Allaah had prescribed. Plural marriage is not only for this ummah (nation – i.e., the Muslims). The fact that the Christians do not allow it may be for two reasons:
1 – This is one of the laws of Allaah which was binding before the time of Muhammad (peace and blessings of Allaah be upon him)
2 – They introduced their own innovation and went to extremes in this matter, as in the case of monasticism which they invented and which was not prescribed for them, but they sought to earn the pleasure of Allaah thereby.
(Islam Q&A Fatwa: 10469)