Ibn Al-Qayyim on Gender Mixing (Ikhtilaat)
Ibn al-Qayyim (may Allaah have mercy on him) said:
There is no doubt that enabling women to mix with men is the basis of all calamities and evils, and it is one of the greatest causes of calamity that affects everybody, as well as being one of the causes of corruption in public and private affairs. Mixing of men with women is the cause of a great deal of immoral actions and zina, and it is one of the causes of widespread death and ongoing plagues.
Al-Turuq al-Hukmiyyah (p. 407).