The Quran allows the marrying of up to four wives if a man believes that he can treat them fairly.
Surah 4: 3. If ye fear that ye shall not be able to deal justly with the orphans, Marry women of your choice, Two or three or four; but if ye fear that ye shall not be able to deal justly (with them), then only one, or (a captive) that your right hands possess, that will be more suitable, to prevent you from doing injustice.
4:3 Dan jika kamu takut tidak akan dapat berlaku adil terhadap (hak-hak) perempuan yatim (bilamana kamu mengawininya), maka kawinilah wanita-wanita (lain) yang kamu senangi: dua, tiga atau empat. Kemudian jika kamu takut tidak akan dapat berlaku adil, maka (kawinilah) seorang saja, atau budak-budak yang kamu miliki. Yang demikian itu adalah lebih dekat kepada tidak berbuat aniaya.
But it says in another place that there is NO WAY you are able to treat your wives fairly.
Surah 4: 129. Ye are never able to be fair and just as between women, even if it is your ardent desire: But turn not away (from a woman) altogether, so as to leave her (as it were) hanging (in the air). If ye come to a friendly understanding, and practise self- restraint, Allah is Oft-forgiving, Most Merciful.
[129] Dan kamu sekali-kali tidak akan dapat berlaku adil di antara istri- istri (mu), walaupun kamu sangat ingin berbuat demikian, karena itu janganlah kamu terlalu cenderung (kepada yang kamu cintai), sehingga kamu biarkan yang lain terkatung-katung. Dan jika kamu mengadakan perbaikan dan memelihara diri (dari kecurangan), maka sesungguhnya Allah Maha Pengampun lagi Maha Penyayang.
Muslim response
Yusuf Ali writes, “Legally more than one wife (up to four) are permissible on the condition that the man can be perfectly fair and just to all. But this is a condition almost impossible to fulfil.”
Christian response
The verse does not say that being fair to all your wives is almost impossible for fulfil. It says as a fact it is impossible. If man cannot have more than one wife unless he can be fair to all (Surah 4:3), and it is impossible for man to be fair to all his wives (Surah 4:29) why does the Quran contradict itself and still permit man to do so?