I don’t think anyone’s disputing that reducing risk with condoms will change behaviour. As is said upthread, that’s one of the reasons for opposition to condom use. The outrage at the Pope comes more from the fact that reducing risk with condoms will also reduce the spread of HIV. It’s a bad idea not to acknowledge that.I thought I had posted a further comment, but it got lost on the Internet.
I remain mystified about what the comment on my comment meant. AIDS is spread in Africa by multiple overlapping sex partners. If condoms make people feel safer having sex, they will be more likely to have more overlapping sex partners, and hence they may undo some or all of the effects of condoms making each sexual encounter less risky. The situation is similar to the question of whether driver's education reduces teenage fatalities. Driver's education makes teenagers better drivers, but it also increases the number of teenage drivers on the road. The second effect can more than offset the first effect.
One way to express these ideas is in the framework of moral hazard. In this framework, things that reduce risk such as insurance reduce the costs of certain events, and therefore people do not have to be as careful about avoiding those events. Another way is in terms of complements and substitutes. Driver's education and teenage driving are complements, so making driver's education more available will also increase teenage driving. Are condoms and sexual intercourse outside of monogamous marriage substitutes, complements, or unrelated? A lot of advocacy of condom use seems to be based on an implicit assumption that they are unrelated. If they are complements, the push for condoms may have consequences quite different from those that are expected. It may, for example, increase AIDS rather than reduce it. The same argument, by the way, holds for teenage pregnancies.