I am not in favor of banning abortion in most cases though, like Chessic Sense, I have moral objections to it in most cases. The fact is, even with exceptions, without a great deal of specificity that gets overly complicated from an external perspective, it’s just impossible to draw any meaningful lines. Moreso, I simply don’t believe in legislating morality. Besides, it’s sort of a silly discussion anyway because the very few abortions actually fall into the exceptions or, on the other end, the cases where the vast majority of people would see it as utterly morally repugnant.
Anyway, specifically to address the OP, I can sort of understand the exception for rape, which is simply that a woman has already been violated and then gets further punished by having to be the mother of a child of not only a criminal, but of someone who did such a horrible thing to her, and asking her to care for that child and love that child is something that is hard for society to ask of her and something that is perhaps difficult to do. How can she look at the child and not so often be reminded of how he was conceived?
So, to that end, I understand the exception, especially when it’s in a case where she gets raped, reports it to the police, and upon discovering her pregnancy, terminates immediately. It would be a lot more difficult to justify if she was raped and waited some period of time before terminating. But still, I would think that anyone who truly believes life begins at conception still ought to have trouble weighing that burden, as great as it is, against what they perceive as murder.
The incest exception makes less sense to me. Someone who does support that explained to me that it has a lot to do with the higher risk of birth defects and such, but then shouldn’t it be an exception just for birth defects? Why should a child, conceived in incest, who shows no significant signs of genetic problems be allowed to be aborted for that risk, when a non-incest conceived child who clearly has a terrible birth defect and will certainly either die in birth or shortly thereafter not be allowed to be aborted?
But even then, legislating on that becomes very difficult to justify. Certainly, I don’t think many would object to aborting a child who would be born without a brain, but what if a parent finds out a child will have a perfectly survivable defect like missing a leg or mentally impaired? If we legislate a line too far in one direction, we may end up putting parents through hardships and pain and expenses for no reason, and if we push it too far to the other, we’re essentially encouraging eugenics.
Anyway, I think that the logic is supposed to be to allow for those very serious types of birth defects that, rightly or wrongly, they believe are significantly more likely in cases of incest.
And, of course, beyond all of that, I think it’s largely an attempt to create a broader stance. I think even a lot of pro-choice people consider the idea of using abortion essentially as birth control and late term abortions are both morally repugnant but support it because they think that it makes sense for a handful of cases and it shouldn’t be banned for those people who need it because of others who misuse it, along the lines of a quote I often heard “legal, but rare”. As such, I think it’s largely an attempt to recruit people along those lines, to say that they really aren’t all that different because, afterall, most pro-lifers are primarily against the same things and while they may even find the exceptions morally objectionable, I think they’d probably rather allow those and ban what they see as some of the greatest injustices than allow all of it.