(This post is nominally about the “withdrawal method” of birth control but is actually intended as an examination of some common pro-life arguments.)
People who believe abortion is wrong often adopt a potential life theory, e.g. that it’s wrong to kill a developing fetus just because it will otherwise develop into a human being. A potential life theory has to contend with a line-drawing problem: what potentialities are sufficient to trigger a moral duty? An easy example: are two people required to have sex, because the possibility of their conceiving a child together equals a potential life? Most people would answer “no,” even if they otherwise accept a potential life theory. One distinction they might cite is the action-inaction distinction: it’s wrong to actively destroy a potential life, but it is not wrong to forbear from creating a potential life, by inaction. Although I find this distinction unconvincing myself, I have long accepted it as an explanation of how potential life theories work. However, I now think it is less coherent than it seems.
However, consider this counterexample: the “withdrawal method” of contraception involves (or can involve) withdrawing the penis at the very last moment before ejaculation. At this point in intercourse, ejaculation is a spasmodic and basically involuntary action — to do nothing is to ejaculate and thus (perhaps) participate in the creation of a life. To withdraw, by contrast, is a willful decision that must be undertaken intentionally (and determinedly). So here the course of action will prevent a potential life from coming to fruition, whereas inaction will allow a potential life to continue towards fruition. It seems, then, that if the action-inaction distinction was the correct explanation of the line-drawing in the potential life theory, potential life partisans would view the withdrawal method as immoral, just like abortion. However, this seems not to be the case. Almost no one thinks the withdrawal method is immoral, so far as I can tell.
Therefore, the action-inaction distinction is a poor fit to explaining potential-life-theory opposition to abortion.