You point out very correctly both the inconsistisies of our thinking and of fashion, and that would be true pretty much at any time. There is no reason to expect that fashion would ever be logical. But there is always some exaggeration in our comments about they way things used to be.
It is true that in the 1960s and 1970s, as well as earlier, to be sure, swimwear for men was often very brief, but not invariably so. Many men never, ever wore shorts, either in public or in private, yet that was at a time when boys and younger men did wear very short and tight shorts, especially cut-off blue jeans. In other words, we are remembering only what we ourselves wore at a certain time, totally ignoring the rest of the world. Kids today are worried about what we are thinking about the way they dress, no more than we did 40 years ago.
I do have to take exception of the nit-picking of what nudity is. It is certainly true that you may not be nude if you were wearing boots, a baseball cap and a sweatshirt, yet you would not dare walk down the street like that. The correct expression is "indecent exposure," a suitably vague expression with lots of elasticity but pretty much everyone has an idea of what it is when they see it. There is still the problem of the way indecent exposure might be acceptable for one person, yet not another. What the movie starlets wear on the red carpet in Hollywood wouldn't go over too well in Buffalo.
I also never saw that many see-through tops in 1970, a few to be sure, yet not enough to count and I still haven't seen any thongs or g-strings on the beach. Maybe I'm not going to the right beaches. But I'm also no longer in decent shape to appear in public on the beach without plenty of cover. Yes, I'm that old now.