A question rarely asked is the one I raise in this post:
Is the abortion question tied to religion in such a way that opposition to abortion can be based only on religious premises?
Or are there good reasons to oppose abortion that are nor religiously based, reasons that secularists could accept? The answer to the last question is plainly in the affirmative. The following argument contains no religious premises.
1) Infanticide is morally wrong.
2) There is no morally relevant difference between (late-term) abortion and infancticide.
Therefore
3) (Late-term) abortion is morally wrong.
Whether one accepts this argument or not, it clearly invokes no religious premise. It is therefore manifestly incorrect to say or imply that all opposition to abortion must be religiously-based. Theists and atheists alike could make use of the above argument.
Now suppose someone demands to know why one should accept the first premise. Present this argument:
4) Killing innocent human beings is morally wrong.
5) Infanticide is the killing of innocent human beings.
Therefore
1) Infanticide is morally wrong.
This second argument, like the first, invokes no specifically religious premise. Admittedly, the general prohibition of homicide -- general in the sense that it admits of exceptions -- comes from the Ten Commandments which is part of our Judeo-Christian heritage. But if you take that as showing that (4) is religious, then the generally accepted views that theft and lying are morally wrong would have to be adjudged religious as well.
But I don't want to digress onto the topic of the sources of our secular moral convictions, convictions that are then codified in the positive law. My main point is that one can oppose abortion on secular grounds. A second point is that the two arguments I gave are very powerful. If you are not convinced by them, you need to ask yourself why.
Some will reply by saying that a woman has the right to do what she wants with her own body. This is the Woman's Body Argument:
6) The fetus is a part of a woman's body.
7) A woman has the right to do whatever she wants with any part of her body.
Therefore
8) A woman has the right to do whatever she wants with the fetus, including having it killed.
For this argument to be valid, 'part' must be used in the very same sense in both premises. Otherwise, the argument equivocates on a key term. There are two possibilities. 'Part' can be taken in a wide sense that includes the fetus, or in a narrow sense that excludes it.
If 'part' is taken in a wide sense, then (6) is true. Surely there is a wide sense of 'part' according to which the fetus is part of its mother's body. But then (7) is reasonably rejected. Abortion is not relevantly like liposuction. Granted, a woman has a right to remove unwanted fat from her body via liposuction. Such fat is uncontroversially part of her body. But the fetus growing within her is not a part in the same sense: it is a separate individual life. The argument, then, is not compelling. Premise (7) is more reasonably rejected than accepted.
If, on the other hand, 'part' is taken in a narrow sense that excludes the fetus, then perhaps (7) is acceptable, but (6) is surely false: the fetus is plainly not a part of the woman's body in the narrow sense of 'part.'
I am making two points about the Woman's Body Argument. The first is that my rejection of it does not rely on any religious premises. The second is that the argument is unsound.
Standing on solid, secular ground one has good reason to oppose abortion as immoral in the second and third trimesters (with some exceptions, e.g., threat to the life of the mother). Now not everything immoral should be illegal. But in this case the objective immorality of abortion entails that it ought to be illegal for the same reason that the objective immorality of the wanton killing of innocent adults requires that it be illegal.