I found an interpretation on another website that gets the meaning of this song perfectly. I want him to get credit for his clever insight so have pasted his comments below:

This is just a theory, but it's a lot more poetic than the "cheating girlfriend" story. How many of Simon and Garfunkel's song's lyrics are so explicitly on the nose? Not many. St. Cecilia is the patron saint of musicians, so Simon is singing about having writer's block. Everything is going great (he's making love) and when he takes a short break (getting up to wash his face) he finds that his inspiration has left him. But it comes back.
- Keith Dennis, Portland, OR