If a corpse was truly no longer a human to us, we would have no problem hanging it up on our walls like game or throwing it into a ditch. Except we don't. We respect our dead by providing funerals and/or proper burials. It's the dignity that we can give them. We give a dead person dignity whether they are human or "used to be human," so conversely shouldn't we give the same respect to fetuses since they are human or "will be human," as you argue?
EDIT: Saying "If we give the same respect to a fetus the same we do to our dead, then it's all right to immediately take it, embalm it and bury it" is a cheap way out and is clearly not what I mean.