Impotency is a medical problem, Viagra is a cure for that medical problem. Yes, some people abuse it (and some doctors enable the abuse), but that shouldn't be a reason to refuse to provide the drug. People abuse Morphine as well, but we're not discussing banning that drug.
Birth control pills are not (except in a few rare cases) treatment for a medical problem. They are a drug which enables a lifestyle choice. I can't imagine any compelling reason why health insurance companies should be required to carry that drug. Why not require health insurance companies to cover condoms and abortions while we're at it?
Bottom line, this shouldn't be an issue for the presidential debate. Health insurance companies should (although it probably won't happen) be able to cover any drugs or treatments that they want. Let consumers decide what level of care they want from their health insurer. I would personally opt for no coverage under $100,000, then 100% after that. The premiums would be miniscule on such a plan.
Actually, pregnancy is an altered state of the body which leads to many health problems for some women. It is not officially an illness, but it certainly heightens risks for certain illnesses including high blood pressure, gestational diabetes, depression, and even death, especially in the absence of proper pre-natal and hospital care. As such, birth control could be viewed as a preventative against such potential illnesses. In the same way that the regular use of aspirin can prevent a heart attack, the regular use of birth control can prevent many pregnancy related illnesses, especially in women who are at risk (35 and older).