Don't worry about being irregular when you go off the pill. Whenever I am off by even 6 hours (which hopefully doesn't happen often), my menstruations are all wacked out, and usually show up a week early!!

This being said, I am on the pill, very physically active, and have extremely light menstruations (I use a whooping total of 2 light-flow tampons and 4 pantyliners for a cycle). Libido is not an issue though... Your mileage may vary. I think we're all built differently...