The main point here is that I dislike abortion, but I now find myself powerless to argue that it is morally wrong when I discuss this with my fellow secular humanists. My dislike about abortion does not reflect utilitarian ethics, but it is merely my subjective opinion.
Unfortunately, if one does not believe in God and in absolute truth handed to mankind by God, then morality has a strictly human origin, is pliable, and subject to either the majority opinion or to the whims of whomever holds sway over a people. I would agree that strictly from an agnostic perspective, it would be hard to argue against abortion in a world where the human species is maintaining a healthy population. But, if there is no God, no afterlife, and thus, no eternal consequences and no real meaning to anything other than what we create in our own minds during our brief existence, then it doesn’t really matter whether abortion is morally acceptable to you, or me, or anyone else. It is difficult to argue any morality in the absence of God.
However, that is not what I believe. As a Catholic, I would interpret your reaction to abortion as evidence of God’s influence on you, or, if you will, evidence of God’s very existence. He infuses us with a sense of the moral law so that most people “just know” when something is simply wrong, even in the face of modern attitudes that would tell us otherwise. I often wonder at the phenomenon I would term “the dead end of self indulgence.” I see so many people in a form of denial about their own mortality, trying desperately to drown out the despair with entertainment, work, pleasures, etc. only to ultimately find that self indulgence is hollow and empty. On the other hand, I see people who sacrifice, suffer, give to others, and live lives of self giving, who face death with peace and joy. These people have found something.
I heard a scientist quoted recently. I can’t repeat the exact quote but he basically said that in the laboratory when exercising his mind, he simply cannot believe in God. However, when he goes into the mountains and looks around he “just knows” that God exists. Reason (the mind) is only part of our make up, a part of the “whole person.” We use our mind analytically, but we don’t use it to express our love, just like we can’t approach a scientific problem with emotion. However, some things engage the whole person - the mind, the emotions, the will, the body and, if you will allow, the soul or spirit. When we “just know” something, I believe that we are engaging the whole person, which might explain why you are put off by abortion even though you cannot make a reasoned argument against it. We are meant to encounter God with the whole person. When we try to encounter Him with our minds alone or our emotions alone, we fall short.
Sorry for the rambling. I hope I’ve said something useful.