Doesn't "birth" mean anything? Sure there's a continuity between the fetus and the neonate, but a 3-month old fetus is still a very different thing from an infant fresh out of the womb. Why not be worried about spilling one's seed? Every one of those billions of spermatozoa is half of a potential life. And all those tampons flushed daily down toilets contain material of potential lives.
For most of Western history, abortion was not a problem. It didn't become a problem until a bit more than 100 years ago. I think Yoshie's right - the "hardness" is a social construction; it has nothing to do with the issue (or the non-issue) itself.
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CB: I agree, and there is no more of a slippery slope between outlawing fascistic racist speech and outlawing socialist speech than there is between supporting abortion and supporting infanticide. Both lines are very easy to draw. The "hardness" of distinguishing fascistic racist speech from other speech is a social construction.
CB