Medically speaking, fetuses do not have the brain faculties to be considered "people" because their minds and self-consciousness do not yet exist. Legally speaking, even if they were medically sentient, they cannot be granted protection under the U.S. constitution because in order for that to happen, it must be a citizen of the United States. To be a U.S. citizen, one must be born in America, or pass a citizenship test and become naturalized. A fetus is obviously not born yet, and is incapable of taking a citizenship test. Therefore, a fetus is not a citizen of the United States and has no legal protections.
If you're a citizen, one of the rights you get is the right to your property, which includes your body. No one can be forced against their will to donate blood, marrow, or other organs to a sick person who needs them, even if it will save that sick person's life. Blood donations require consent forms, for example. They can't just drag you away in a van, suck out a pint or two, and toss you back out on the road. In the same way, no woman can be forced to give up any part of her body, blood, nutrients, or organs to a fetus. Banning abortion by making fetuses "people" is probably one of the most legally disastrous things I have ever heard, because it grants a fetus more rights than any other person in the United States - the right to take away your right to property.
[/Lawful Neutral]
I'd like to point out though that most people I know who are against abortion even in cases of rape do not actually believe the woman is responsible for what happened to her. They argue that the fetus should not be punished for the rapist's crimes. Granted, it completely misses the point of abortion in the first place, but fortunately I've never found the "lol ur a hore" mentality particularly common in everyday life here.