The central problem is a question of whether human life is sacred.
I mean sacred in a pedantic, dictionary sense: set apart. Special. Treated differently then others of its kind. A sacred chalice is just a cup: the only difference is that it’s a cup we set aside and treat differently than all other cups. To desecrate (ie, de-sacred) the cup you don’t have to go so far as to piss or shit in it: you could just drink kook aid out of while eating KFC. Both desecrate the sacred cup by treating it the way we treat all other cups, as a tool we can do whatever we wish to.
Traditionally in Western culture (really in almost all cultures but I’ll focus on the West as I know it best) we have treated human life as sacred. That is, it is treated differently than all other living things. To kill a mans dog means little more than breaking his table as far as the law is concerned: to kill the mans child is murder, and requires the murderers execution. “He who sheds the blood of man, by man shall his blood be shed” had been a central pillar of Western law for thousands of years. Today many balk at executing men, but it is argued out of a sense that human life is more sacred, not less: that killing a man for murder is as evil as murder itself, in some sense. Animals we may decide to treat better or worse, but animals are not sacred, no matter how smart they are. Human life is sacred.
And I think this cultural value is important and we should not cast it aside as superstition so easily. It may seem arbitrary from a secular view but all values are arbitrary from a secular view. You posit quaila as the determining value of life: yet someone might easily posit another. Why not intelligence? Why not kill the feeble minded, why are their lives sacred? Why not physical ability? Why not kill the cripple, why is their life sacred? Why not race? I don’t want to Godharts law myself, but it’s right there: the Nazis agreed that not all human life is sacred. Stalin and Mao agreed as well: the lives of reactionaries were not sacred, nor of oppressor classes or the wit decedents.
We toss away the idea that all human life is sacred to our own risk. It took thousands of years to refine “Man is made in the image of God” into human rights, universal suffrage, and the abolition of slavery. Abortion, by claiming that not all human life is sacred, threatens to turn us down darker path.