There was plenty of male nudity in '70’s movies. One of the scenes that really cracked me up in Monty Python’s Life of Brian was Graham Chapman opening that window shutter in the morning totally naked and being startled by all his followers outside. It was just something about being in a most vulnerable moment and suddenly confronted by a pack of loonies who think you’re the messiah.
The MPAA got really prudish going into the '80’s and pretty soon just showing a schwanzstucker could get an X-rating or later the dreaded NC-17. I’ve read that Porky’s had it’s own battles with the MPAA on how many seconds you could see a certain body part in the infamous shower scene.