Why free mixing of sexes is not permitted according to scriptures?

Because woman is like a fire and man is like a butter. When they come together butter melts.

So as a result illicit sex. As a result bad kids. Because in order to have good kids one should be married properly and perform garbhadhana samskara before having a child. So it should not be as animals do. 

There is not friendship between man a woman. When they come together only attraction is sex. So because of that shastras restrict this free association. 

Whole world is moving around this desire. Example, take all movies if the actress is not cute enough and if there is no so called loving relationships among man and woman no body will watch that movie. So it is the root cause of why we are here in this material world. 

And it is the strongest rope which tights us here to this material world. 

Separately both man and woman are good but when they are connected improperly then problem arises.