Obviously, bicycling is _not_ bad for your bones. But being a competitive road racer type bicyclist, where you exercise a ton and eat as little as possible and try to whittle your upper body down to the point where you're just barely strong enough to lift your bike IS bad for your bones.
Female Athlete Triad: it's not just for women! Men, too, an give themselves early onset osteoporosis by exercising too much and eating too little. Surprise.