The worst type of men and women are those who talk openly and freely with no need to the opposite gender, as they are causing a great fitnah which effects the entire Ummah.
Shaykh Islām Ibn Taymiyyah (رحمه الله) said in “Kitāb al-Istiqāmah” (1/361):
“The mixing of one gender with the other is the cause of fitnah. When men mix with women, it’s like mixing fire and wood.”
Shaykh ‘Abdullāh Al-Sa’d said:
“Free-mixing is forbidden by Sharī’ah, whether in education or work, and in all private and public gatherings. There have never been any era for people of Islām, mixing their women with foreign men.”
Allāh says in the Qur’ān:
وَلَا تَقْرَبُوا الزِّنَا ۖ إِنَّهُ كَانَ فَاحِشَةً وَسَاءَ سَبِيلًا
“And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way.” [17:32]
Meaning, as Ibn Kathīr (رحمه الله) says in his “Tafsīr” — Allāh is forbidding His servants to commit Zinā, or to approach it or to do anything that may lead to it as it is a major sin and causes immorality.
It’s also mentioned in “Al-Tafsīr wal-Bayān Li-Ahkām il-Qur’ān” (4/323-324), commenting on Sūrat al-Ahzāb, Āyāt 32-33:
“Then do not be soft in speech [to men].”
Meaning: Do not commiserate with him (i.e., speak in a compassionate remorseful way), even if that was done out of good intentions, for indeed the prohibition is not for her alone, but for the listeners, so it would incline and tempt whomever has a veil and sickness in his heart towards them (i.e., women), so they would be the cause of his destruction.
Furthermore, Ibn al-Qayyim (رحمه الله) said in “Al-Turuq al-Hukmīyyah” (page 407-408):
“Mixing of men and women is the cause of a great deal of immoral actions and zinā. And it is one of the causes of widespread death and ongoing plagues. One of the greatest causes of widespread death is the prevalence of zinā, because of enabling women to mix with men and walk among them displaying their finery and beauty. If the people in charge knew how much corruption that (free-mixing) causes in worldly affairs and people’s affairs, let alone religious affairs, they would be stricter in putting a stop to it.”