The question we are trying to determine here today, is just when does it become a person. As I have pointed out we can not determine the answer to this question, neither through science nor through Christian scripture. However in other cases where we are unsure, we usually pass laws that error on the side of safety. So what is it exactly about this issue that makes it any different?
Well, it's not clear that we do err on the side of safety when it comes to metaphysical propositions. We don't forbid the eating of animals in case they have souls, or of bacon in case Allah disapproves, or of beans in case Pythagoras was right.
We don't mandate human sacrifice to the Mayan gods in case the world will end in 2012 if we don't.
We don't have laws against adultery or taking the Lord's name in vain just in case God will smite the unregenerate nation in which these things are lawful.
In fact, I am at a loss to know what you
do have in mind when you say that we "err on the side of safety".
If it comes to that, we don't allow unlimited abortion on demand just in case God is fanatically pro-choice and will punish the unbelieving nation that interferes with a woman's Sacred Right To Choose, either.
You are candid enough to say that we can't find our answer in Christian scripture --- but in fact it is only with reference, if not to Christian scripture, then at least to conventional Christian ideas that you get your notion of which side safety might lie on. You say we should "play safe", but you (perhaps unconsciously) assume that when we're playing it safe, the only metaphysical mandates that we might have to worry about as potentially existing and that therefore might demand our caution are those of the American Religious Right.