Why don't insurance companies have to cover birth control for medical issues?

Here's a question that has often occurred to me: why is Viagra covered when birth control, which is often prescribed to treat conditions such as polycystic ovary disease, is not by many insurance companies?

Sandra Fluke, a Georgetown law student, testified before Congress this month about the financial hardship the lack of insurance coverage imposes on a sexually active female law student there. By her calculations, the Pill will cost such a student an additional $3,000 per month during the three years of law school. Georgetown University is a Catholic institution. Fluke also testified about a friend who needed the pill for medical reasons.

There has been an ongoing debate about whether religious-affiliated institutions will have to fund insurance coverage of contraception under health care reform. Opponents argue that the requirement will infringe on their religious beliefs.

I have my doubts that birth control pills for a medical condition are covered even if the condition has been diagnosed by a doctor, but that seems quite unfair when it, like erectile dysfunction for which Viagra is prescribed, is a medical issue. Viagra is arguably not any more necessary than the pill, since both men and women can abstain from sex or pay out of pocket for a pill if they so desire. But I knew of several young teenage girls in high school – not yet sexually active – who were prescribed the pill for various medical conditions and I suspect that medication wasn't covered by all insurance plans either. When it's for a medical condition, I would think that even Catholic-affiliated institutions should pay for coverage.

Personally, I think contraception should be covered as a matter of course by health insurance even when it is specifically intended to prevent pregnancy, since that is a routine health care need for most women of child-bearing age, but most definitely when it is for some other underlying medical condition.