You are right of course. It always comes down to what constitutes life. And of course, that is where we part ways.
Where I find a inconsistency is how murder, as a "moral wrong" can exist in a world that was allegedly an accident, with no purpose, and us killing each other would merely be animals thinning the herd, just because it is what we do.
If you believe in God as the creator of the universe, then I would ask, "how do you know?," and "is it worth the risk of being wrong?"
As faith-driven as it is, and I freely admit it, I can appeal to a higher power. The most science (in the hands of an a-deital individual) can do is appeal to the intellect of man, and no man's opinion is therefore worth more than that of another.
In my opinion this is the basic crux of the argument.
Abortion is murder if life begins at conception.
According to my understanding of the Catholic faith, life begins even before conception. Therefore contraception = murder (hence the Catholic position against contraception).
Typically, the argument is made that life begins at conception. This argument is usually based on religious grounds. This is consistent with the views posted in this thread thus far.
Another typical argument is that life begins at some point after conception (i.e. viability). This is more of a scientific view.
In any case, it is not clear when exactly life begins. That said, no religous opinion has a place in this argument.
"Is it worth being wrong?..." It is fine for you to use this argument to decide for yourself not to have an abortion... but if my religion doesn't say that life begins at conception, then your religion has no place telling me what to do.