Imaam Ash-Shaqeeti (rahimahullaah) said:

Allaah made it a common phenomenon that men are fewer in number than women in the various regions of the world and are more often exposed to the causes of death in all walks of life.  If a man was restricted to one wife, there would remain a great number of women deprived of marriage, so they (both men and women) will be strongly urged towards illegal sexual intercourse. [See Adwaa al-Bayaan’ Surah Al-Israa Ayah 9]