People hold contradictory and incoherent views on abortion precisely because they don't really believe the unborn are fully human, despite their rhetoric to the contrary. As Dr. Francis Beckwith points out, "Why do women only kill their fetuses when confronted with practical difficulties rather than their already born children if they truly believe their fetuses are human?"
I agree with Scott in that too often, women in unplanned pregnancies fail to internalize their stated beliefs. How often do you read post-abortion testimonies where a woman starts off by saying, "I used to be prolife until I got pregnant...." If someone really believed and fully internalized that the unborn were human beings and killing them was an objective moral wrong, then how does one's own personal situation change those objective facts? Living with that kind of cognitive dissonance has to be extremely difficult.
I think that cognitive dissonance leads many people who would otherwise be prolife into the pro-choice camp. When faced with the idea of "I ended the life of my child" vs. "Abortion is a tragic decision but it doesn't kill a real human being" it seems much easier to side with the latter.
"Only bad people kill their children. I'm a loving mother who cares for her children. I had an abortion. Therefore, abortion can't take the life a human being because I had an abortion and I'm not a bad person." Or "I know women who've had abortions and they're not bad people. Only bad people would end their child's life. Therefore, abortion doesn't really take a life."
Some women spend years fighting objective facts because their personal experiences don't comfortably jive with them.
I also think the legality of abortion is also something that preys on the minds of women who say they believe the unborn are human beings yet have abortions in spite of their beliefs. Justifying thoughts like, "If the unborn were really full human beings, then how could abortion ever be legal" and "If abortion is legal then it can't really be killing a human being. Our government wouldn't allow that," begin to creep into the minds of women who fear the possible repercussions of being pregnant out-of-wedlock and/or can't imagine raising a(nother) child.
Don't we also see this same kind of lack of idea internalization with regards to abstinence before marriage? How many Christian teens (while not in a relationship) have made pledges to abstain from sex until marriage because they recognize God's gift of sex is meant to be shared between married men and women but then reject this pledge once they are in a relationship where sex is a possibility? Then how quickly does the cognitive dissonance of having sex outside of marriage while believing sex outside of marriage is wrong turn into believing that sex outside of marriage is perfectly alright or not that bad?