Hello! Your post about what you do at the gym made me think of this, and you being so active and needing a healthy, strong body brought me to mention it. One of my ballet instructors is currently in school to become a physical therapist and she teaches at local fitness centers as well (Zumba, aerobics, water aerobics, Pilates, and yoga) and she said that sit-ups are actually really bad for your back. They're damaging to your spine more than they benefit your abdominals unfortunately. :/
Any physical exercise can damage something, especially if not done properly. However, to say that sit ups are bad for healthy individuals is wrong.