Imam Ibn Qayyim (may Allah have Mercy on him) said:
“There is no doubt that enabling women to mix freely (ikhtilat) with men is the root of every calamity and evil. It is one of the biggest reasons for general punishment (from Allah) to befall (a society). It is also one of the reasons of corruption of general and specific matters. Free-mixing between men and women is also the reason for increase in immorality and illegal sexual intercourse (zina). It is one of the reasons of widespread death and incessant plagues.
One of the greatest reasons of widespread death is fornication and adultery (zina) due to enabling women to mix freely with men and move among them made up and dressed up. Had the rulers known what corruption it causes in worldly affairs and the society, before the hereafter, they would have been the strictest in stopping it.” [Al-Turuq Al-Hukmiyyah, page 407-408]