In every discussion regarding abortion, it is highly likely that the pro-death side will resort to the saddest possible example to try to guilt you into supporting it. I remember in college, I was speaking to two of my friends about how a girl had recently blocked me because I was against abortion. A girl standing nearby got immediately angry, shouting "if a 13 year old gets raped, of course she's going to have a f***** abortion!" But I didn't say anything about 13 year olds getting raped. I merely said I was blocked for being pro-life. What the nearby girl had done was come up with the saddest possible example she could, then used that to justify abortion as a whole. But that doesn't make logical sense. You can't say 13 year olds get raped, therefore the promiscuous 25 year old who sleeps with more men that she can count is also entitled to an abortion.
To give a similar example, I believe it is morally acceptable to kill a man who is attempting to rape said 13 year old girl. My belief that it is ok to kill in a defensive scenario does not mean I also believe it would be ok to kill the 13 year old girl. Killing in defensive situations is an exception to an otherwise well established rule. In the same way, if we accept that rape is an acceptable reason to allow abortion, that still doesn't justify abortion as a whole. Even the New York Times, a notoriously Left wing news publication, admits that abortion due to rape and incest combined make up about 1% of all abortions (1). And of course, it's not always actual, or forcible rape. Sometimes it's statutory rape (i.e. where the victim gave consent when not legally considered able, such as when intoxicated, or below the age of consent). Sometimes, the mother just doesn't want to admit to consensual sex for fear of judgement, or even punishment. So even the reported statistics may be inflated.
So now we've established that exceptions, if they exist, do not affect the rule, and we've established that rape, if we grant it, would be an extreme exception. So why do abortionists argue this way? They do so because arguing the rule doesn't carry the same emotional weight. If you point to a 13 year old rape victim, you can pull out the violins and make it seem like the pro-death position is somehow the compassionate one. But the argument with regard to the rule is "my body, my choice". No violins, no tears, just complete and utter selfishness. That might win a few converts, but it certainly won't convince anyone who possesses critical thinking skills and a moral compass. Thus, arguing from possible exception is by far the most effective way to win. Nevertheless, it remains fallacious, and can only be used by the ignorant and malicious. If you've ever used this argument, ask yourself, which one were you? Dumb or evil?
References
1. Lewin, Tamar - Rape and incest: Just 1% of all abortions, The New York Times, 1989