One of the many problems with this statement is that it is, quite simply, wrong. It is as wrong as the medieval belief that bad smells (miasmas) cause illness. As wrong as the Renaissance medical system of humors. It is not even debatably wrong, like the beliefs of climate change deniers. It is simply, 100 percent incorrect.
Like the "dunking stool" used in the Salem witch trials, pregnancy is - for some people - the indication that a woman was not "really" raped. For example, if a woman was wearing a short skirt or was a little bit tipsy, that - in their opinion - means that it was not a "legitimate rape." How can you tell? Because she got pregnant.