It's the "moralistic fallacy," the idea that we should shape the facts in such a way as to point to the most morally desirable consequences. In the case of rape, the fear was that if rape has a sexual motive, then it would be natural, hence good; and instinctive, hence unavoidable. Since rape is bad and ought to be stamped out, it cannot come from "natural" sexual motives. My own view is that these are non-sequiturs -- rape is horrific no matter what its motives are, and we know that rates of rape can be reduced (in Better Angels I assemble statistics that US rates of rape are down by almost 80% since their peak). One surprise that I experienced upon re-reading Susan Brownmiller's 1975 book "Against Our Will," which originated the rape-is-about-power-not-sex doctrine, is that idea was a very tiny part of the book, thrown in almost as an afterthought (Brownmiller said she got the idea from one of her Marxist professors). Most of the book is a brilliant account of the history of rape, its treatment by the legal system, its depiction in literature and film, the experience of being raped and reporting it, and other topics. It's also written with great style, clarity, and erudition. Though I disagree with that one idea, I would recommend it as one of the best and most important books on violence I have read.
