Death.

> It follows that many of those who died from AIDS would still be alive today had the Catholic Church abandoned its condom ban in the 1980s, as it became clear that condoms were (and still are) the most effective practical measure to prevent the spread of the disease.

http://en.wikipedia.org/wiki/Catholic_Church_and_HIV/AIDS

Also forbidding sane family planning and the prevention of unwanted offspring, where lack of necessary resources causes even more disease and more death.

People undeterred by the catholic church's prohibition on sex outside marriage are unlikely to be reluctant to wear condoms because of doctrinal concerns, I'd have thought.

Theoretically, yes, but sadly the reality isn't quite like that. Passion/desire often trumps doctrine, and doing something that one thinks one shouldn't do can have all sorts of nasty consequences, ranging from teen pregnancy to untreated STD's.

Having been an Evangelical Christian and 'assistant teen pastor' in a previous life, I've seen this in action in many different areas of life. People constantly did things they believed were wrong, and the combination of not being prepared or informed about those things and feeling ashamed about it afterwards led to a lot of suffering.