I would agree if one is writing about Victorian sexual politics straight-up, however I was careful to specify the sexual politics of Victorian literature. For which Freudianism, notoriously wrong as it is, is highly relevant because it was enormously popular for a chunk of the time period, and did directly influence writers (more particularly in the early 20th century than the late 19th, but still). Certainly it had much more influence than post-Victorian science that the authors could not possibly have been aware of.

Which seems to me one of the hedges that postmodernism usefully offers. The decision to approach Victorian literary sexual politics in terms of the thinking of the time and to treat it as a phenomenon of that culture is, to my mind, quintessentially postmodernist.