Looking at abortion rationally, without invoking God.
There are two related questions, with four possible answers. Question one is: 'is abortion murder?' The second is: 'do we know for a fact whether it is, or whether it isn't murder?'
If abortion is murder, and we know it is, then there is no excuse, it is murder.
If abortion is murder, but we don't know it is, then killing the child is manslaughter.
If abortion isn't murder, and we don't know it isn't, it is like a hunter shooting into rustling bushes; where they might get their deer, or they may kill a person, too. This is essentially a form of criminal negligence.
Finally, if abortion isn't murder, and we truly know it isn't, then abortion is perfectly justified.
Without having definitive proof (scientific/legally accepted proof, that it) that the child is a person, or that it isn't, we are left with only two possibilities: manslaughter or criminal negligence.
So, it is really on the abortion advocate to scientifically prove that the pre-born children they plan on killing aren't persons.
To answer the question, 'why would we want to discuss abortion without invoking God?'
A: To save the holy innocents.