No Kirsten, you are not being overprotective. It just proves to us how much you love your furbabies cause you want to keep them safe from harm.
Not all cats are light on their feet. My Mookers would never be able to sit in an open window(not that he would be allowed to anyway) as he is one clumsy kitty. When he would jump onto my dresser, everything went flying whereas my Katie girl was so graceful that nothing would be out of place.

So it is best to be safe than sorry, and good for you for not allowing your kitties to sit in an open window.