Even if you don't sit, you still have to be bent over to some degree, both in the hips and in the shoulders (to reach out for the handlebar), which is a position physiologically similar to sitting in front of a computer (sitting in a chair and reaching out for the keyboard). Also, the actual activity of cycling tends to stress the hip flexors, which are often already chronically contracted due to prolonged sitting and doesn't activate glutes that much (unless you use those cycling shoes which make the upward movement of the feet contribute to the bicycle movement as well). This is exactly the opposite of the pattern of activity that people with sedentary lifestyles need. This imbalance of muscular tension that I'm talking about results in very posture problems, like the anterior pelvic tilt - http://healthhabits.files.wordpress.com/2008/05/anterior-pel...

You're obviously well educated on the subject. I agree with what you're saying. That's why my recommendation above was to do Bootstrapping Squats (at first), bench, deadlifts and chins -- just the basics. You especially want to strengthen the Posterior Chain (hamstrings, glutes, lower back) and Bootstrapper Squats are a simple way to get started.

There's a great book called Starting Strength that gives a basic starting program.