A number of "pro" riders, both male and female, have been forced into early retirement by weak bones. But, it is unlikely that RIDING is the cause of weak bones. The cyclists with weak bones have other factors...genetic factors...their diet...lack of "load bearing" exercise.
Problems with weak bones come from not having enough calcium in the diet, and from not doing enough "load bearing" exercise. Doing eight or ten hours a week of brisk walking or jogging will improve bone density in the legs and hips. Lifting weights will improve bone density in the upper body.