For those with the time and training, it's best just to read the refreshingly clear exchange between Frs. Martin Rhonheimer and Benedict Guevin. (It's a PDF; if you don't have Acrobat Reader, you will be prompted to download and install it. Trust me, it's not hard.) For those without both the time and the training, here's the upshot.
The issue is whether condom use for the sake of disease prevention within marriage is an instance of contraception, which the teaching of the Church condemns as "intrinsically evil." I say "within marriage" because the Church's teaching about contraception presupposes that the sexual act in question is morally licit, at least in itself and prescinding from contraception. To my knowledge, the Church has never taught that the aim of limiting the damage potential of illicit sexual activity is intrinsically—i.e., always and necessarily—evil. Even granted that passing out condoms to the promiscuous is likely to make them more so, that's only an argument against adopting condom use, as opposed to incentives to abstinence, as a church or state policy to combat the effects of promiscuity. It does not tell us anything decisive about the case of any individuals in particular. So the sole issue is whether a married couple exercising what used to be called their "conjugal rights" may ever use, as a means of preventing transmission of a serious disease one partner is known to have, something that also has a contraceptive effect.
It is granted all around that the "subjective" intention of a couple should not and need not be contraceptive. But granted also that condoms are fairly effective contraceptives regardless, can it be said that the "objective" intention of the couple is contraceptive when they use a condom to block STD transmission? If the answer is yes, then contraception is being intended as a means to a further end, and hence even prophylactic condom use is immoral according to the irreformable teaching of the Church. But Cardinal Martini argued, and I agreed, that the answer is no. Since the contraceptive effect of the condom neither acts as nor is intended as a means to the end of prophylaxis, the principle of double effect as traditionally invoked in Catholic moral theology allows for condom use in this case.
Fr. Guerin, Luke Gormally, and others, however, disagree. They argue that since condoms are contraceptive by their nature, inasmuch as that is one of the purposes for which they are produced and which they usually attain, using a condom during vaginal intercourse can be said to embody an intention to contracept even if one doesn't make contraception one's conscious aim. My rejoinder is essentially the same as Fr. Rhonheimer's: such a stance unduly stretches the concept of intention. Just as the Church does not condemn a woman's using an anovulant pill for purely therapeutic purposes, even though one of its effects is contraceptive, so the Church should not condemn a man for using a condom to protect his wife from his AIDS, even though one of its effects is contraceptive. Abstinence of course is better as both personal and public policy. But that doesn't rule out a second line of defense, at least for the individual.
Rumors abound that the Pope is preparing to rule on this question. I state in advance my resolution to adhere, with religious submission of mind and heart, to whatever he does rule. But I rather doubt he's going to take sides in this debate. He will reiterate the Church's teaching against contraception. That will be good and necessary. He will point out that distributing condoms to all and sundry is not going to solve the problem of sexually transmitted diseases. That will also be good and necessary. He will urge abstinence on people with AIDs, which is only prudent and is certainly the better course. But I doubt he will make the position of the Guerins and Gormallys the definitive teaching of the Church. The arguments need time to grind against one another. And that too will be good for the Church, if only because it will dispel a misimpression that Catholic dissidents love to exploit.