While we're on the subject, I note the following from the August 2006 Harper's. This is an excerpt from "The Rhythm Method and Embryonic Death," by Luc Bovens, published in the June 2006 Journal of Medical Ethics:
It has not gone unnoticed by pro-life advocates that if one is concerned about abortion because of the moral turpitude of killing embryos (and fetuses) then one should also be concerned about contraceptive techniques — the morning after pill, intrauterine devices and the contraceptive pill — that cause embryonic deaths. Catholics might welcome this, since the official position of the Church is that, aside from the rhythm method, no contraceptive techniques are permissible. What has gone unnoticed is that the rhythm method may well be responsible for massive embryonic death.
Rhythm-method users try to avoid pregnancy by aiming at the period in which conception is less likely to occur and in which ovum viability is lower. So their success rate is due not only to the fact that they avoid conception but also to the fact that conceived ova have reduced survival chances. Just like in the case of pill usage, we do not know in what percentage of cases the success of the rhythm method is due to the reduced survival chances for the conceived ovum. Nonetheless, one could argue that even if the mechanism has only limited effectiveness, it remains the case that millions of rhythm-method cycles per year globally depend for their success on massive embryonic death. Even a policy of practicing condom usage and having an abortion in case of failure would cause fewer embryonic deaths than the rhythm method.
Bovens isn't entirely fair to the Catholic Church — its opposition to contraception is not based entirely on "the moral turpitude of killing embryos," but also on arguments involving God's providence and sovereignty. These latter arguments are rather severely undermined, if not contradicted outright, by the insistence of many within the Church that the rhythm method, scrupulously applied, is more effective than the forbidden methods. (This supposed greater effectiveness ought therefore to constitute a greater supposed insult to providence.)
If reliance on the rhythm method does, in fact, result in massive embryonic death, then consistency would seem to require that the most strident advocates of saving the lives of early embryos ought to be opposed to the use of this method. Yet instead we find that this method is vocally endorsed and promoted by this same group. Odd. How to explain this inconsistency?
Update: Belatedly checking my e-mail, I see that Gordon W. sent me a link to the complete Bovens article several days ago. Thanks G.W., and here's that link: "The Rhythm Method and Embryonic Death."