With Ganymede Quartet, I tried to make all the things that weren’t slavery in the 20th century as historically accurate as possible, and that included the language I used. Here are some words that would have come in handy but were not appropriate for the time period.

• “Sexy.” You don’t realize what convenient shorthand this word is until you can’t use it. Modern usage of sexy dates from the 1920s. Apparently “sexful” would have been all right in 1900, but I wasn’t about to refer to any character as sexful for what I think should be obvious reasons!
• “Asshole” as derogatory term/insult. Calling people assholes didn’t see common usage until the 1930s.
• “Jerk” as a noun/derogatory term, though it could be used as a verb, as in “jerk off.” Referring to people as jerks also dates from the 1930s.
• “Wank.” Dates from around 1950.
• “Come” as a noun, as in the product of ejaculation, though it could be used as a verb.  The verb form actually dates from the 1650s, but referring to semen as come is as recent as the 1920s.
• “Blow job.” This one really wasn’t a surprise. Dates from about 1948.

A gentleman by the  name of Jonathon Green, creator of an English language slang dictionary, has made extremely handy timelines of sex terms, which you can access from his page. Very useful for historical smut :)

Penis, vagina, and alcohol terms: http://jonathongreen.co.uk/timelines/
Intercourse terms: http://jonathongreen.co.uk/timeslines-continued/