It should be mandatory for women on active military duty to use an IUD or Norplant (or be sterilized, if they prefer). Exceptions should be made on a case by case basis, based on the woman's function in the military and the military's needs for the foreseeable future. We don't have a draft, so nobody would be forcing women to use contraception or be sterilized. But if you sign up for a job like this, you have to be prepared to ensure you'll be fit to perform it.
Also, I'm certain the courts would reject this policy if women already under contract weren't grandfathered. Unless given the option to re-sign or get out, that is.