I first discovered "The Mary Tyler Moore Show" when I was 25, and I immediately fell in love with it. But a few years ago, when I showed some episodes to a family studies class I was teaching, my students were far less impressed than I had expected them to be. I explained what a revelation it had been in 1970 to see a show about an unmarried working woman in her early 30s who was devoted to her job, was attractive to men and well-liked by her fellow workers, and who turned out to be the most…