Why can Muslim men have up to four wives?
Because the Koran says it is permissible, since Mohammed himself had at least 11 wives and many sex slaves. One of his wives, Aisha, married him at the age of six. For a Muslim, Mohammed is the greatest example to follow. That is why it is allowed for a Muslim to have up to 4 wives.
Who were Mohammed's sex slaves?
Mohammed's sex slaves were non-Muslim women conquered by Muslims. The rape of Kafir woman is established in Islamic Law and in Islamic Traditions. Abusing women - much like car bombs, nightclub shootings and random stabbings - is a manifestation of the assault on Western values. The purpose is to create confusion and fear.
If a sex slave is married and has a husband, it is still morally right for a Muslim to have sex with her:
4:24 Also forbidden to you are married women unless they are your slaves. This is the command of Allah. Other than those mentioned, all other women are lawful to you to court with your wealth and with honorable intentions, not with lust. And give those you have slept with a dowry, as it is your duty. But after you have fulfilled your duty, it is not an offense to make additional agreements among you. Truly Allah is knowing and wise!
The above verse was created at the time of the jihad at Khaybar. In that period, Mohammed attacked the Jews of Khaybar and conquered them. The Jews that survived were doomed to become dhimmis . They had to cultivate the land and were forced to give half of the proceeds to the jihadists. In addition, some women were taken as sex slaves .
Sex slaves in Europe
Rape as a tactic of jihad has been used in many countries for centuries and now we are witnessing it in Europe as well. Many Muslims believe that raping women is morally good and take it as a reward for jihad. Repeated violent attacks on women in Europe have contributed to the rise of movements that oppose these incidents. The biggest of them is called #120db . Because young German girls are victims of Muslim immigrants, they have started to raise their voices and have pointed out the issue that the media and government across all Europe are trying to play down and keep quiet .
You can find more about jihad and sexual slavery in the book of Bill Warner, The Doctrine of Slavery, Center for the study of Political Islam, 2010.
To have sex with slaves is, for Muslims in the same moral category as being humble, telling the truth or giving a donation to charity. There is no blame associated and it is a moral good since it is allowed by the Koran.
70:22 Not the devout, who pray constantly and whose wealth has a fixed portion set aside for beggars and the destitute, and those who believe in the Judgment Day, and those who fear their Lord’s punishment—because no one is safe from their Lord’s punishment — and who control their sexual desires, except with their wives or slave girls, with them there is no blame; but whoever indulges their lust beyond this are transgressors, and who keep their trusts and promises, and who tell the truth, and who are attentive to their prayers. These will live with honors in Gardens.
Jihad was sometimes very ugly, as the following verse describes:
I980 When Zayd had raided the Fazara tribe, he and others were injured. Zayd swore he would never have sex until he had avenged his injuries. When he was well, Mohammed sent him against the Fazara. He was successful and captured some of the women. One of them was an old woman, Umm Qirfa, whose husband he had killed. Zayd tied a rope to each leg of Umm Qirfa and tied each rope to a camel and pulled her apart. Her daughter was taken and passed around to three different men to use as they would for their pleasure.
Comprehensive information about polygamy with precise quotes from the Koran, Hadith and Sira can be found here.
 A dhimmi is a non-Muslim, who submits to the rules of Islam.
 Bill Warner, The Doctrine of Slavery, Center for the study of Political Islam, 2010.