After watching The Last Duel (where great emphasis is put on this 'fact') I struggle to believe this was true. In a time where war and the rape of women on the losing side was common I find it hard to believe people genuinely believed any resulting pregnancies meant she enjoyed it. Is this another Hollywood inaccuracies?
/u/sunagainstgold here informs us (in an answer with a somewhat different focus) that it was indeed widely recorded in medieval sources that women needed to orgasm to get pregnant, with the implication that you describe:
"...more disturbingly, because Christian thought held that orgasm was the voluntary result of lust, pregnancy was held up in court as proof that rape had not occurred--the woman had committed fornication or adultery with her rapist instead, since pregnancy proved orgasm which proved she wanted it."
Though I'm sure an answer with a closer look at those sources would be fascinating too, I can't find anything more specific.
In 1755, a rape victim by the name of Sarah Robertson was cross examined in a London court. On being asked by the defendant's lawyer if she was afraid of falling pregnant as a result of the rape, she replied that it was impossible "for where a woman has no inclination, it is impossible there can be a child." This was something she knew because she had been "a married woman a great many years", and shows just how long this idea - originating in the 5th century BCE Greece - survived for. In fact, it survived in legal circles down to at least 1788, where Samuel Farr stated it as fact in Elements of Medical Jurisprudence, although by then it was an outdated idea from a medical and legal perspective and was gone in English legal circles by 1792. Two centuries later, it's an idea still present in modern society.
Let's go back a bit, to the 5th century BCE. Hippocrates, born c.460 BCE, was the first physician that we know of to have applied a scientific method to science and established a famous medical school on the island of Kos. Although he may not have written any of the texts attributed to him, many of these date to the time when he was alive and were almost certainly written by his students. One of these is the text know as On Generation, which describes the conception process.
To summarise, On Generation maintains that both men and women produce seminal fluid, which combines to make the foetus. The author regards an orgasm as necessary for both sexes to produce this fluid but, in my reading of the text, doesn't allow that women might not experience any pleasure from the act of sex at all. Instead, the author writes that women can make both "seeds" leave her body "whenever she desires".
The Hippocratic author, although not the only school of thought on the subject of conception, was ultimately the most influential author on the subject of conception to the medieval mind because the Roman physician Galen accepted his theory and Arabic medical understanding was firmly rooted in Galen. Arabic translations of Galen's writings, or those written by authors adapting Galen with their own observations, were translated into Latin and made their way into Western thought during the 12th century.
William of Conches, while tutoring the future English king Henry II between 1146 and 1149 produced Dragmaticon Philosophiae, an influential work in the world of medieval science. In it he argued (following Galen) against Aristotle's "One Seed" theory and emphasised that physical pleasure was necessary for conception. However, and here he is either following the 2nd century CE Greek physician Soranus through some unknown chain of transmission or has coincidentally arrived at the same conclusion, William also allowed that while a woman might not "assent with her rational will", her "natural" body might nonetheless experience pleasure and allow her to conceive.
This distinction should have argued against the idea that if a woman falls pregnant she has actually consented, as should the complete refutation of the idea that a woman needed to experience pleasure to conceive by Ibn Rušd (Averroës), but late 13th century and early 14th century law codes seem to have latched onto the idea that pleasure was necessary for conception and could only occur if the woman was consenting.
For instance, the late 13th century English legal text known as the Fleta states that if "the woman should have conceived" the charge of rape must be dropped "for without a woman’s consent she could not conceive." This in turn influenced English common law and remained down to the 18th century as mentioned earlier. While England was an early adopter of the idea as a legal opinion and other places in Europe had far more localised versions of customary laws, the idea women couldn't conceive unless they consented was widespread in Europe by the 15th century and continued strongly into the 17th.
While there were examples of women who had become pregnant by rape successfully prosecuting rape charges in Germany in spite of these laws and beliefs, it was nonetheless extremely difficult and there are no examples from England or France. The case of Lady Isobel Butler, from 1436, is rather depressing and telling. She was raped, abducted, forced to marry her attack, raped again and was only rescued when her friends alerted the King's Commission and her attacker fled before he could be arrested. Beyond her initial appeal, there are no contemporary accounts of the prosecution, but references to the case during a 1465 trial and a collection of Inner Temple Readings from the end of the 15th century indicate that she likely lost her case as a result of the pregnancy resulting from rape.
I haven't watched The Last Duel and I can't comment on exactly how they portray the concept, but it must have been something Marguerite and Jean de Carrouges were worried about. Perhaps fortunately for them, Jacques Le Gris never made the accusation and instead relied on denying that the rape had even happened. Had there been enough witnesses it might have become a factor in the case, but as there were no witnesses apart from Marguerite and Le Gris' cronies it was easier for him to just deny everything. It would in any case have been difficult for him to prove that the child was his rather than Jean de Carrouges', given the natural variations in gestation and a sometimes shaky medieval understanding of this length.
As to what medieval women, especially those who were called on to examine rape victims and testify in court, knew, there's not much we can say. Medical texts by women, such as the Trotula and Hildegard of Bingen's Causes and Cures are silent on whether or not a rape victim can become pregnant, and there's evidence that midwives and other women involved in healthcare did talk to doctors and that information flowed both ways. It's entirely likely, as in the case of Sarah Robertson, that many held to the belief that it was impossible to become pregnant if raped.
On the other hand, it's clear from Ibn Rušd's investigations that at least in the Islamic world many women were well aware that you could become pregnant without experiencing physical pleasure, whether within a marriage or as a result of what was considered rape, and it's entirely possible that this was known to experienced midwives or other women who were involved in rape cases. Unfortunately, due to a lack of sources, this isn't something we can ever know with any degree of certainty.
Recommended Reading
Ann Ellis Hanson. “Hippocrates: ‘Diseases of Women 1.’” Signs, vol. 1, no. 2, (1975): 67–84
Elise Bennett Histed, “Medieval Rape: A Conceivable Defence,” The Cambridge Law Journal 63 (2004): 743–69
Eric Jager, The Last Duel
Hiram Kümper. "3. Learned Men and Skillful Matrons: Medical Expertise and the Forensics of Rape in the Middle Ages". In Medicine and the Law in the Middle Ages