Does Islam promote polygamy?
Polygamy in Islam is permission not an injunction.
Historically, all the prophets except Jesus, who was not married, had more than one wife. For Muslim men to have more than one wife is a permission which is given to them in the Qur'an, not to satisfy lust, but for the welfare of the widows and the orphans of the wars. In the pre-Islamic period, men used to have many wives. One person had 11 wives and when he became Muslim, he asked Prophet Muhammad, "What should I do with so many wives?" and he replied, "Divorce all except the four." The Qur'an says, "you can marry 2 or 3 and up to 4 women if you can be equally just with each of them" (4:3). Since it is very difficult to be equally just with all wives, in practice, most of the Muslim men do not have more than one wife.
Prophet Muhammad himself from age 24 - 50 was married to only one woman, Khadija. In western society, some men who have one wife have many extramarital affairs. In Islam, it is seen as more honourable for a woman to be a second wife than the 'other woman'. She has legal right and respect in society which she wouldn't do otherwise as a girlfriend/mistress.
Does Islam oppress women?
On the contrary, Islam elevated the status of women 1,400 years ago by giving them the right to divorce, the right to have financial independence and support and the right to be identified as dignified women (Hijab) when the rest of the world, including Europe, had no such rights. Women are equal to men in all acts of piety (Qur'an 33:32). Islam allows women to keep their maiden name after marriage, their earned money that they can invest or spend as they wish, and asks men to be their protector as women on the street can be molested. Prophet Muhammad told Muslim men, "The best among you is the one who is best to his family."
However, some Muslims warp Islamic teaching seemingly to oppress women in the name of "religion" an aberration deriving from cultural habits or ignorance about their religion. Female Genital Mutilations (female circumcision) has nothing to do with Islam. It is a pre-Islamic African custom practiced by non-Muslims including Coptic Christians.