I'm no expert on pregnancy, rape, nor on "right to life" thinking. I have, however, been intrigued about any possible back story on the concept of "legitimate" rape, and the claim that women have some sort of biological mechanism to prevent pregnancy in case of such rape (which is total nonsense). A lot of theories have been advanced to explain why such nonsense is perpetuated – something amiss with male psychology; perhaps there is some deviant male will to dominate that seems self-evident on the coservative side of the political spectrum. There's also been speculation about the perverse fascination for categorizing different levels of force where rape is concerned – "forcible" rape, "legitimate", "consensual" rape, etc. I've read some articles about these ideas which often theorize without clearly delineating a likely motive. I find most of these arguments unpersuasive.
I now think a different theory seems more plausible – we don't need to explore male psychology or pathology to understand why some promote this nonsense; there is, rather, a dark, pathetic, yet very straightforward explanation that fills the bill. It springs from the will of the so-called pro-life community to curtail abortion by whatever means. I note that Todd Akin has not just support, but also total agreement from the likes of the American Family Association and from the Family Research Council, and these endorsements appear to bolster this simpler explanation:
Women who have been persuaded they have an innate defense mechanism against pregnancy in case of rape are less likely to seek medical help, and are therefore more likely to forego any sort of pregnancy and/or abortion counselling. Ergo, fewer abortions.
This social impact of the likes of Todd Akin's bungled propaganda was noted by Dr. Jill Powell, a gynecologist at St. Louis University. Her comments were added (almost as an afterthought, it seems) to an article that explored how heavily right to lifers continue to rely upon now discredited sources. For those who appreciate the more arcane twists of bizarre "legitimate" rape trauma/pregancy shutdown theorizing, the article quoting Dr. Powell in Missouri's own St. Louis Post-Dispatch also explores the Nazi death camp roots of the concept.