The argument against abortion is that even though we don't know when a human being becomes a human being, and therefor deserves the right to life (even while depending upon another's body for that life), we should respect all human beings enough not to risk their lives on the question.
The argument on the other side is not an argument in favor of abortion, it's an argument in favor of individual autonomy and self-determination. And at what point in a human being's development this individual autonomy becomes extant, such that it competes with the mother's. And then how we resolve those competing autonomies should they conflict.
Each argument is essentially alien to the other, which is why these debates never find resolution, or even common ground: the debaters simply talk past each other on completely different paths of thought.