"Had the Pope signed on to condom distribution in Africa, he would have done nothing to reduce the spread of Aids, but he would have done a lot to advance the further artificial separation of sex, in Africa and beyond." – Mark Steyn (via Hispanic Pundit)
Economic theory gives no clear prediction. Condoms lower the chances of transmission of VD from any one encounter, but they also change incentives resulting in more encounters and multiple partners. And even if a particular individual's number of encounters and partners does not change, in all likelihood their partner is on average likely to have had more encounters and more partners than before the promotion of condoms.
For more see Philipson and Posner, Private Choices and Public Health: The AIDS Epidemic in an Economic Perspective.