Brit Hume starts the conversation, but does not dig deeper (so we will):
http://youtu.be/zEs4pD9Dn14 (if it cannot be embedded)
First, if we know where babies come from, then we also know where the "choice" is to be made-- and was made.
Second, if we do not know when a human life begins (Feeling pain? Consciousness? Incarnate soul?), then we err on the side of caution.
Third, as a pregnancy out of wedlock is potentially two very negative things: A traumatizing social stigma and and an economic disaster. Morally and ethically, we do not get to choose to murder (see second item) to avoid these negatives.
Fourth, the baby also recognizes the father's voice! That baby is every bit the choice of the father as it was the choice of the mother (see first item).
Fifth, "conformation bias" when making a decision when in a social or economic crisis (see third item) is not necessarily forever. That is, women who have chosen to abort, often accuse themselves of murder afterward. And that has devastating traumatic effects upon the psyche.
Ask any priest, ask any psychological counselor, ask any therapist. An abortion is one of the most common self-traumatizing regrets heard from women.
In other words, if we do not know if the fetus is a human person or not, and make a decision we later regret in the case of abortion, that action is equivocal to murder which is a far greater trauma to carry than social and economic trauma.
So it is that, outside of a spiritual life, the typical reaction to choosing an abortion is to accept, as FACT, that the abortion did not take a human life. Guilt avoidance will not allow but the most introspective to even begin to consider that one perpetrated an unimaginable horror on another for the shallowest of reasons-- the ultimate betrayal of their own nature.
This then, leads to a person's determination to deny that anyone has a soul, that anyone rightly has spiritual thoughts, that anyone matters-- including themselves.