But is there any evidence that this phrase was ever used during Victoria's reign? Apocryphally it's sometimes attributed to Victoria herself, although this tendency has abated as Victoria's enthusiastic enjoyment of sex has become better known. Other than that, the earliest citation appears to be from the 1912 journal of Alice, Lady Hillngdon, in which she expresses her relief that these days she is obliged to "lie down on [her] bed, close [her] eyes, open [her] legs and think of England" only twice a week. But that quotation first appeared in print in the 1970s, and the journal itself (as Brewer notes) has never been produced, so it must be treated with suspicion at best. Something similar is said to have been given as advice to her daughter by the wife of Stanley Baldwin, but again, no evidence.
It's tricky, of course, the subject matter being such that people who might have conceivably have used the phrase would have shied from putting it down in print; but surely there would have been many women from the early to the middle part of the twentieth century who would have attested to its use in earlier times? Did the Suffragettes make no mention of it? Marie Stopes? Virginia Woolf? Gwen Raverat? Marie Lloyd? Anyone at all?
By this point I'm pretty much convinced that the phrase is a twentieth-century invention, foisted on the Victorians as a way of poking fun at them (c.f. Victoria refusing to outlaw lesbianism) - but I'd still like to know who first came up with it.