I've talked in depth with a religious cousin of mine about this issue, and in our case it clearly came down to a difference of religious beliefs. I think that this is really just another religion vs. science debate.
Specifically, this is a manifestation of the ongoing
mind-body debate. For those of you unfamiliar with it, this is the question of whether our mind, our self, our personhood, our consciousness, etc. is linked to things only in the body or if something else (ie: a soul) is involved. Now, from a scientific perspective, this debate is fairly settled. We now have a decent understanding of how consciousness and personality and all that are directly connected to the human brain.
I think you see where this is going. For me and other scientifically-minded people, a zygote is merely a potential human life. Why? Because it has no mind. It has none of the defining characteristics we think of when we think of people. It can't think, it can't feel. At what point that begins is up for debate, but it isn't at conception.
For those who give religious beliefs priority over scientific ones, life begins at conception because that is when the soul is created. How? Why? No explanation; it's just faith. So for them, abortion is murder, plain and simple.
True, it has the potential to become a human life if left alone. But an egg or a sperm cell also has the potential to become a human life. If you're really going to argue that this potentiality is of vital importance, but not when you have to intervene in some minor way to realize it, I'm going to have to disagree. Personally, I find this agrument to be absurd. It is the same argument as this, however, that makes it somehow OK to take someone off of life support in certain situations, but
never legal to actively assist someone if they choose to die, no matter how close they are to death. Even if it means they have to live a few more agonizing, miserable hours, or even days. I think the euthanasia debate is related in this way and I find both of these opposing positions quite absurd. Indeed, it is quite unfortunate that they carry so much weight with some people.
edit: grammar. And btw, being pro-choice isn't being pro-abortion. Come on, now. No one is saying we need more abortions. Of course abortion isn't plan A. You use protection -- not just to avoid pregnancy, but to avoid transmission of STDs. An abortion is costly and there is some danger to the woman's reproductive health. It is always regrettable, even for those who don't believe that zygotes are people.