Joseph, perhaps you think that I am missing the point, but I believe otherwise.
I think that it was the cautious familiarity between Jewish men and women (which was the norm) that enabled them to interact with one another in a permitted way. It is precisely because now men and women are kept so far apart that they have no idea how to interact with each other when happenstance brings them together.
I am not encouraging frum people to let their boys and girls mingle so that they will know how to deal with members of the opposite gender. I am saying that once separations are so rigidly enforced, those separations must be enforced for life or they will act like teenagers no matter what age they are, no matter what their marital status.
It is precisely because young men and women DID have interaction with one another in your grandparents’ time (with properly established boundaries, but less rigid, of course) that they had a realistic understanding of the opposite gender and were able to act appropriately.