As a more moderate religious conservative, I take the unusual stand that a fetus is not a person from the point of conception, but rather, sometime soon after.  In an interesting twist, Luc Bovens (London School of Economics and Political Science, Department of Philosophy, Logic, and Scientific Method), writing in the latest issue of the Journal of Medical Ethics, accuses that NFP actually is abortifacient.

In The rhythm method and embryonic death, he argues that having sex when a woman is less fertile still results in some fertilizations, but they don’t implant – just like when you take the pill.  Anti-evangelical site talk2action has a nice post on this.

Professor Bovens contends that a massive switch from birth control pills to NFP "would be effectively causing the deaths of millions of embryos."

Similarly, regular condom users, whose choice of contraception is deemed to be 95% effective in preventing pregnancy, would "cause less embryonic deaths than the rhythm method," he says.

"[T]he rhythm method may well be responsible for massive embryonic death, and the same logic that turned pro-lifers away from morning after pills, IUDs, and pill usage, should also make them nervous about the rhythm method."