Doesn't that assume that they really do have a logically consistent position? And without meaning to sound tediously partisan, I generally find that this isn't very frequently the case with people espousing a pro-life politics. There arguments have a pretty strong tendency to display a lot of clumsy and poorly-examined assumptions about conciousness, about personhood, about ethics, even about biology, more often than not the result of trying to secularise a logic originally constructed within the terms of religious belief. (And let's get it straight, there are legitimately secular dualisms. You could try to make a Cartesian defence of a pro-life position, for example; it would actually be very interesting. But that is not what these people do, because their theology is rarely empirical and rational, and very commonly mystical and irrationalist.) Granted, pro-choicers are often guilty of many similar points of insufficiency, but their logic is generally unpolished rather than simply incoherent; a second- (or third-, or fourth-)hand copy from a more coherent thinker, rather than a semi-literate translation between two different conceptual frameworks. They are making arguments badly, rather than making bad arguments.
Now, I suppose you could argue that this is consistent within certain limits, but that's not the same thing as simply being consistent. (If a foetus really is a person from Day One, then, yes, it's logically coherent to regard abortion as murder, but on what basis are we making that initial assumption?) And if we're only allowed to address the aspects of each others positions that are unassailable coherent, then, well, there's really no such thing as philosophical debate any more, is there? There's just a lot of people saying "well I suppose you're entitled to your opinion" in a very insincere tone of voice.
I'm perfectly willing to acknowledge that a person has an internally coherent position if they have one. And, biased as I may be, none of those pro-lifers in possession of such a position- and I know they exist, because I've read their arguments- seem to have the energy for these debates.