I cannot recall, from my days as a conservative evangelical, anyone holding to the belief that women who are raped are unlikely to get pregnant. But it wouldn't surprise me if it were widely held, and undiscussed even amongst those who the people holding the belief live and work with. People aren't dumb, in two ways:
1. People recognize they need to have some sort of empirical justification for the (normative) views they hold. Mostly due to a lack of imagination, these explanations have to be scientific, or 'scientific.' (I.e. there are other ways to assert empirical truth other than concocting a biological explanation, but these are out of vogue for a number of reasons.) Scratch any strange-seeming political or social belief for long enough, and there's an explanation to be found.
2. People recognize these explanations are crazy, or will seem crazy, and so greatly restrict the sphere in which they're discussed. It's entirely possible to have a discussion of what's wrong with abortion amongst a evangelical crowd without having recourse to biology; it's possible to discuss it as part of a policy debate without ever referencing these beliefs.
This approach isn't limited to evangelicals or abortion. The Catholic traditionalists I used to know would eventually admit that one of the reasons birth control was (morally) wrong is because it is a dangerous drug that can kill women, but that belief is unlikely to come up unless really pressed on the matter. Nor is this restricted to the right, or religious people: instructors (who have generally not taken economics of any kind) tend to decry the empirical ignorance of their students (who generally have) if that ignorance leads them to want to put Marx aside as irrelevant: Marx asserts a truth that's truer than standard economic explanations of what's wrong with his theory. (Granted, I also think this, but one has to let Marx take his lumps on those things about which he's wrong.)
The usual response to these beliefs cropping up is to mock them. However, as J.S. Mill would tell you, that's the wrong thing to do. People who are mocked are not going to change their minds; they will learn to hide their beliefs better. Changing beliefs requires patient explanation without condescension. Of course, people have difficulty managing this, and it takes a lot of time, which is why it rarely happens. But it's the only way it can happen.