In 2007, my friend Michael Patton of Reclaiming The Mind ministries authored a thoughtful piece here entitled "Can a Christian Support Abortion? The Theology of Abortion". He is absolutely correct in affirming abortion is ultimately a theological issue, pointing out that one's position on abortion is the inevitable result of their personal theology.
While you may not have ever consciously thought through your own position on the creation of the soul, your theological understanding (whether conscious or subconscious) determines your position on abortion. I personally believe that Michael is correct in affirming Traducianism - the view that holds the soul comes into existence at the moment of conception. As Michael point out,
Theologically speaking, it is impossible for there to be a Christian traducianist who supports abortion. Why? Because the traducianist’s theology precludes a necessary belief that a person is complete from the moment of conception.
Ultimately he says,
Our stance concerning the issue of abortion is not our guide with regards to this theological issue. In other words, we do not choose the position that best fits with our agenda one way or another. We must seek to find the truth, not defend our preconceptions. (emphasis is mine)
As I've said before, if I cannot be 100% sure and have some uncertainty pertaining to the theology of an issue, I will always err on the side of caution because of the potentially devastating consequences if I get it wrong (i.e., see here.)
Michael's article is must-reading.
I suspect if you ask people why they either support or don't support abortion, that most answers will be based on subjective feeling and/or personal experience. Everyone without exception has a personal theology - whether or not they realize it - that forms their worldview. Unfortunately the worldview of most is not consciously formed from a concerted effort to grasp theological truth, but rather from the whims and dictates of a rudderless culture.