It doesn't sound plausible to me that there has been any point in the last several generations where it was considered "not a big deal" for high school teachers to have relationships with their students. That apparently happened at my high school, once that I know of, during my time there, and it was a very big deal indeed. I can't give you proof that my experience generalizes, but I claim that it does, and that my claim is unremarkable. Whatever you claim the norm was in the 1970s, it clearly didn't survive, which seems fatal to your argument.