Long before I became a Catholic at age 34, I opposed abortion. I came to this position on the basis of reason alone -- as an Evangelical I had not been schooled on this issue. Evangelicals, by the way, did not start their anti-abortion activism until the 80s, taking over leadership from Catholics like
Dr. Jack Willke (one of my personal heroes) who responded so quickly to the
Roe v. Wade decision in 1973.
My reasoning to oppose abortion was initially based upon reading a chapter written on the "contraceptive mentality" by existential psychologist Rollo May and the 'Metaphysics' of Aristotle. From Rollo May I was spurred to think about our attempt to control nature, and from Aristotle I learned metaphysics of act/potency and the difference between 'first nature' and 'second nature.'
At the moment of conception, a human person possesses a 'first nature' containing all the potencies of a human person, potencies that can become 'act' in his or her 'second nature.' In other words, all human persons possess a common first nature, while each of us actualize that first nature in different second natures.
For example, if you compare my first nature to that of Brian Saint-Paul, they are identical -- we both share the same potencies. But if you compare how we have actualized that nature in our second nature, then some serious differences appear, e.g., a golf swing, good taste in music, and the ability to dance :)
I have not read the recent book by Prof. Christopher Kaczor of Loyola Marymount University in Los Angeles, but I know Kaczor and am confident his book is worth reading --
“The Ethics of Abortion: Women's Rights, Human Life, and the Question of Justice.” 
