Also it seems that b_b has considered their actual argument, and has responded to it in his/her post with his/her concerns.
As to your points about slavery, sacrifice, etc. I agree that what matters is how our current society views moral matters, not what generations past thought. Despite what moral absolutists say, history is clear that morals are relative. I think most people would agree that at this time in the West, killing babies is pretty universally abhorred. Plus, on the practical side, good luck getting any doctors (ya know, that whole "First do no harm" thing) to kill a healthy newborn. That would cause PTSD in even the most hard-hearted physicians, I believe. I don't know about Australia, where this paper came from, but pretty much every state in the US has laws whereby a mother can abandon a child at a hospital, or police or fire station, no questions asked. It seems to me that even if one can construct an academic argument in favor of infanticide, there is no practical room for it in a civilized society.
Given that morality is a system of accepted actions that benefit society (which I think is not too much of a leap), one can objectively say that things like slavery, murder, and rape are wrong.
That's the odd thing about the law. It works with the assumption that consistency and logic can be applied, where sometimes it cannot. Some lines just don't seem to be definable. We can recognize the two separate states, but we just can't seem to define the transition. You can make effective arguments all along this issue. This one is interesting, because it seems to test the logic of the extreme. If anything, I think it works to elucidate just how arbitrary these definitions are.