Victor, I think you have a good point, and this is a part of church teaching that isn’t talked about. The common understanding is that the church is against birth control of all forms no matter what, but Humanae Vitae would seem to indicate otherwise.
One has to ask, then, does using condoms to prevent the spread of AIDS in fact cure the disease? Should someone have to contract the disease in order to use a contraceptive to prevent its spread to another person? Seems kind of silly, right?
And even more poignant is your point about married couples – if one has HIV is it morally justifiable to give HIV to one’s spouse because of a prohibition on condoms? Or should the couple have to suffer a sexless marriage? There’s a lot of territory to be explored here.
All in all, I still affirm, with the church, that abstinence is the most effective way to erradicate disease. Places like Africa need a culture change to decrease and eliminate the number of rapes. Rampant sex isn’t cured with condoms, but with culture change, which is ultimately what the pope was advocating for when he said condoms are not effective in the fight against HIV.