The fear of getting hard
The restless mass of lager

The story:

The first time I heard it, I knew those couldn't be the real lyrics, but it powerfully described my first experience of night swimming, on Hampstead Heath, London after an evening drinking at a pub (bar) with the friends of a recently-acquired girlfriend.
I really was affected by two fears - the folly of trying to swim with a belly-full of beer and the fear of getting an erection in front of young women who were "comfortable with their (and everyone else's) nudity" and would expect me to be likewise.
So that, I think, is why I heard it the way I did. But I looked up the right lyrics on the web as soon as I could. The web may be arresting the development of mondegreens.