> The argument for abortion isn't that it's somehow OK to kill really young human

Not true! At least, if you asked me to make an argument in favor of abortion, that's the argument that I would make.

No human within the first year or so after conception has any of the distinguishing features which give intelligent beings a higher moral status: Tool use, language use, abstract thought, logical reasoning, permanent memory.

Therefore, the moral status of a human immediately after the time of conception t_0 is akin to that of an animal. It's not okay to torture, abuse, or injure them for fun; it's not okay for people outside their family to interfere with them; it's morally and socially acceptable and even admirable for individuals to form very deep emotional attachments with them and spend enormous resources helping and protecting them; but it is okay to kill them humanely if there simply isn't a place for them.

The moral status of the very young human rises from that of a proto-human animal to that of a full human at time t_I, the time when they become intelligent.

The problem is that t_I is extremely difficult to measure. It depends on your definition of intelligence and can be very different for different individuals. (For severely brain-damaged individuals, it may even be the case that t_I = infinity.)

To deal with the difficulty of measuring t_I, our culture and laws have instead picked a deadline t_D, such that t_D is much smaller than t_I in all cases (regardless of differing definitions or individual cases). We have declared that we should assume humans younger than t_D have proto-human legal status roughly equivalent to animal status, and humans older than t_D have full-human legal status. Killing before the deadline is legal if done in a humane way with the consent of the mother; killing after the deadline is treated the same as killing an adult would be (only allowable in very limited circumstances such as self-defense). Therefore our society can avoid some of the drains placed on it by humans who, it is known, will be born into far-from-ideal circumstances, while hopefully staying far away enough from t_I to avoid sanctioning the killing of anyone who has the moral status of a full human. (Obviously, t_D is the time of birth plus delta, where delta is less than one day (I'm not sure exactly what delta is, but I'm pretty sure it's less than one day). Delta is necessary to account for partial-birth abortions.)

The problem with many pro-abortion arguments is that they are of the form "It is our belief that all humans have souls, and therefore full-human moral status, from the time of conception." It's not a problem that those are their religious beliefs; the problem is enshrining those beliefs into law in prohibitions on abortion. Logic and facts, not religious beliefs, must be the basis of our laws; to do otherwise would open up the very can of worms that we try to avoid by separating church and state. We can't observe souls, but we can observe intelligent behavior, and the latter seems not to preclude existing abortion laws in any way. (Existing abortion laws also don't interfere with the religious beliefs of abortion opponents; if you believe abortion is morally wrong, you don't have to have one. The law only steps in to the extent that anti-abortionists wish to impose their beliefs on others.)

Hypothetically, if the laws were expanded to legalize infanticide as "extremely late abortion" at any time up to, say, age three, then a moral argument would hold weight; many two-year-olds do use language and can exhibit surprisingly intelligent behaviors. Likewise, if the laws were expanded to require abortions in certain circumstances (for example, to enforce a one-child-per-family policy like China has), that would also be wrong and almost surely unconstitutional; for it would interfere with the religious beliefs of most abortion opponents. (I say "most" since I suppose it is possible there is some faction whose religious beliefs, if any, don't forbid abortion, but bases an opposition to abortion on non-religious grounds. If they even exist, they're surely very small and not very vocal.)

Of course, everyone (myself included) regards such expansions as extremely morally repugnant; I know of no one who would even consider supporting them. I'm just offering them against a hypothetical opponent who would say, "If you would sanction the killing of innocent babies, you clearly have no moral compass!" This worldview does have limitations on what is allowed; it just draws the line between "unallowable atrocity under any circumstances" and "the lesser of two evils" in a slightly different place than pro-lifers.

As much of a tendency as it is among liberals to put all conservatives in the same boat, both on HN and everywhere else the left gathers, abortion is possibly the best place to draw the line between libertarians and social conservatives: The latter favor prohibitions, while the former should oppose them as unwanted government intrusion (although many do not, I suspect, due to simple realpolitik -- a sacrifice of a non-core position by a weaker leg of the stool to support a core position of a stronger leg to maintain the conservative coalition, presumably a compromise to be "renegotiated" -- possibly unilaterally -- when/if their relative strength ever changes.)