Salam Dear Nadeem,
Thank you for your question and for contacting Ask About Islam.
It is good that you and your roommate can talk about religion. Many Westerners do not want to discuss what they consider to be a “personal matter.”
However, in order to guide your roommate to Islam, it would be best if you limit your discussions to matters of creed (aqeedah), according to the practice of the Prophet (peace and blessings be upon him).
The questions you raise have been answered before on this page. I will just give you brief answers here and the links to more detailed answers.
The Islamic practice of polygyny (multiple wives) is often attacked by the enemies of Islam, but you should point out to your roommate that Islam did not initiate polygyny. It was a common practice at the time, and, indeed, is found in many non-Christian cultures both historically and at the present time.
While men and women have equal rights in Islam, they don’t have equal responsibilities. A woman has the responsibility to obey her husband in matters regarding the marriage. What would she do if she had multiple husbands and they gave her conflicting orders?
Islam is also adamant on attributing the child to his or her biological father. If a woman had multiple husbands, she would not know who is the father of her children. (We can’t expect that everyone can afford and have access to DNA testing!)
As for the marriages of the Prophet (peace and blessings be upon him), it must be noted that none of the Prophet’s contemporaries—not even his enemies—criticized him for the number of his wives or the young age of his wife Aishah. What he did was within the norms of his time, and we should not judge his actions by our standards today.
I hope this helps answer your questions.
Salam and please keep in touch.