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The Urban Legends Reference Pages take a look at the "Advice and Instruction for Young Brides" here: http://www.snopes.com/weddings/newlywed/advice.htm
The gist of it: The wording gives it away. Although the use of the word "sex" to indicate the sex act was sort of known in the very late 1800s (it previously had been used only to indicate gender), its use in that form then would have been quite cutting edge. One wonders if a minister's wife would have thrown it about with such abandon. Surely "conjugal relations" would have been the term of choice.
Other language usages give one pause: " . . . and turned off all the lights . . ." Would people in 1894 speak of 'turning off' lights? Usage changes more slowly than the technology around it, and at that time even though electric lights were in use in many households, one would still term the act of shutting them off as "putt...