Why does rape or incest matter?

Samhita at Feministing (one of the greatest blogs in the sphere) takes Sam Brownback to task for his comments on abortion and rape and incest the other night:

Yeah you heard it right. Senator Sam Brownback actually argued on Wednesday that, "We talk about abortion, but abortion is a procedure. This is a life that we’re talking about. And it’s a terrible situation where there’s a rape that’s involved or incest. But it nonetheless remains that this is a child that we’re talking about doing this to, of ending the life of this child."

Now, I grew up in a fundamentalist household, and I've simply never understood the rape-and-incest line of argument, as if somehow whether the fetus is a product of rape or incest changes the fundamental facts of the abortion question.

I feel like I do a better job than most really breaking down the debate, but we'll see.

A fetus is one of two things:

• A) A human being, and should thus have full protection under whatever laws we could scrounge together to protect it, or
• B) A clump of cells living in a woman's uterus that, if given the opportunity, would likely somehow escape the uterus and begin development as a human being.

By tone of writing, its readily apparent that I support the latter definition.

From B), two options seem reasonable to me:

• 1) Mother decides that the clump of cells living in her uterus represents a future human being that should develop as her child. She wants that child, and she wishes for that child to escape her uterus and begin development, and from this point has a legal obligation to protect the child.
  
• 2) Mother decides that the clump of cells living in her uterus does not represent a future human being that should develop as her child. She thus decides to have abortion to have an unwanted mass of cells removed from her body.
  

So, where does rape or incest fit into ANY of this paradigm? I see no morally defensible middle ground between A) and B) or between 1) and 2).

Between A) and B), we could insert these notions:

• i) A fetus is a human being but does not enjoy the rights of a human being outside the womb.
• ii) At some arbitrary point during gestation the fetus changes from being a clump of cells to a human being, irrespective of a mother's wishes.

But neither make any sense to me. i) requires arbitrary notion of a second-class citizen. ii) requires that there's something magical about a cellular process independent of a social construct. I reject both of these outright as absurdities, although I find these are two popularly held beliefs. They're convenient, and let people make decisions about abortion de facto (and set silly regulations about so-called partial birth abortions or legislate abortion regulations differently depending on trimester of gestation) without actually considering any consistency with other moral beliefs. These notions might be useful shortcuts, but they don't seem logically defensible.

So I only see two morally defensible takes on abortion:

• A*) Abortion is wrong because a fetus is a human being who deserves all the rights and protections of law, except possibly in the case of a mother's life being in danger, where all of this paradigm breaks down, and more subheadings would be required.
• B*) Abortion is on average morally neutral and a decision left to a woman who has the right to make decisions about her own reproductive health regarding a pregnancy at any point prior to its termination through abortion or delivery.
  

Again, by tone of writing, its readily apparent that I support the latter definition.

I just don't see how rape and incest fit into this paradigm. Either a fetus is a human being, and killing it is murder, or a fetus is a prospective human being, and killing it is not murder.

I'm a firm believer in B*, but I do believe that A* is morally defensible, even if I absolutely reject it, because I know of no absolute way to differentiate between A and B. Now, in some cases, such as when Sam Brownback starts talking, A seems flat out hateful and ignorant. "You got raped and became pregnant? Tough shit! Go to church, you filthy harlot! They have free nursery during services!"

But I know a handful of rational, pro-women pro-lifers who subscribe to A and A*, and they are intelligent and thoughtful enough for me not to reject their ideas as ignorant. A and A* can certainly be included in a rational progressive agenda that supports women in poverty, provides unfettered access to proper contraceptive and reproductive health resources, and punishes perpetrators of violence on women. But give me B and B* any day, because they simply make more sense to me, and allow women more control over their lives.

I'm actually asking for answers here, and I know of at least a few regular readers that have the backgrounds to make reasoned comments. Why does rape or incest matter when determining whether an abortion should be permitted?