Rabbi we have progressed. Schools are coed, boys and girls dance together at proms, and they enjoy it. Why is traditional Judaism so fearful of early romance and mingling?

Answer:

A wise man once said: “Not everything that is new is good and not everything that is old is mold.”

Judaism values modesty, appreciates that girls should be girls and boys should be boys, and prefers romance when the time is ripe.

It is true modern society has progressed as you accurately describe. But it has progressed further than your description. It hasn't stopped at mingling. It continued into the ugly world of teenage promiscuity, pregnancy, depression, and more than an occasional suicide.

And it doesn't take an Einstein to correlate the obvious connection between the "progression" of coed activities and the increased rates of promiscuity, pregnancy and depression etc.

But since you put your faith in modernity, I will point you to an article written in a most modern periodical. Apparently whilst Judaism's perspectives have been around for a while, they have not expired. These ideas are quite alive in many "progressive" circles as well. 

For example, in the April 16, 2001 issue of Time Magazine there was an article titled Puppy Love's Bite. The article examined the dangers of early romance.¹ It warned:

"A recent study of 8,000 adolescents in the Journal of Health and Social Behavior shows that love can be hell, especially when it strikes young. Although steady romantic attachments tend to increase the health and well-being of adults, those relationships are a great source of stress for adolescents and can lead to depression, the study says... Dr. Miriam Kaufman, a pediatrician and author..., says... "I advise kids not to jump into romances too early, because young adolescence is a time for girls to cement their friendships, and strong friendships--and a strong sense of self--help kids through the tough times".