This "absolute rape" is not quite the same as Akin's "legitimate rape". Akin seems to be suggesting that the body suppresses conception or causes a miscarriage, while the earlier idea of Farr relates specifically to the importance of orgasm. Through the medieval and early modern period it was widely thought, by lay people as well as doctors, that women could only conceive if they had an orgasm.
The biological basis for this idea is what the historian Thomas Laqueur has termed the "one sex system". The one sex system suggests that women's reproductive organs are fundamentally based on men's reproductive organs, so the vagina is represented as an inverted penis, the ovaries are testes and so on. Women had "cooler" constitutions, and therefore lacked the heat or force to drive the gonads out of the body, to convert ovaries to testes.
I don't think medical ignorance has anything to do with Akin's ideas. It has to do with the fact the anti-abortion people do not believe there should be abortion in the case of rape or incest because they believe women would lie about it to get one. It's that simple.
Contortion acts like Akin's or J.C. Willke's peddling absurd claims about rape "inhibiting" ovulation are just smokescreens to cover their hostility toward women. They think "irresponsible" women would make up a rape claim to have an abortion of "convenience."