Well I can give you my perspective from a former Catholic who herself had this opinion, as did many others I know.
These aren't my views on abortion now, to be honest, now, I don't much have much of an opinion about abortion one way or another. I wouldn't consider myself pro life or pro choice. I'm not saying it's not an important issue in a general sense, it's just not one to me, and it's just not one that I think about at all unless it is brought up by someone else.
However, back when I was pro life...
While there are pro life people who think that the fetus' health/life supercedes the mothers, and that it is wrong to abort even a fetus with severe problems/deformities, etc...that's not all of them. I was in the latter group. I always considered the life of the mother to supercede that of the fetus. If the fetus severely endangered her health or life, then I thought her health came first. Same thing if there were serious developmental/genetic issues with the fetus; in such a case, I had no problem with abortion. I also didn't have a problem with the morning after pill or birth control pills. Although I did believe a fetus was a human...I still did view the fetus as a distinct category of human which should not be offered the full scope of rights an actually fully formed human would be, especially not if it would endanger a human who was already here (i.e. the mother). And while I didn't think abortion should be legal as it is, I did not view abortion as the same thing as the murder of a fully formed human.
In the case of rape, I felt that forcing a rape victim to carry her rapists' child against her will would be so potentially damaging to the woman that it would justify an abortion because not doing so would be potentially catastrophic to her mental health. With incest, I kind of applied the same logic as incest is typically from an abusive relationship, with the added issue of risk of genetic defects.
I know you could then say that someone could get pregnant without being raped and, for whatever reason, being forced to carry that child could cause serious mental issues for her as well. But I didn't really think about that much when I held these beliefs, and at that time, I probably would have justified the distinction by saying that well, that woman put herself in her situation so now she has to deal with the consequences, while a rape victim did not.
I'm not trying to defend the reasoning behind my opinions, only explain why I held them in the past.