This is just one of several such terms, including the expression "Pardon my French" to excuse vulgar language.
"It is thought that the term French is employed in this sense as it already had a history of association with things considered vulgar. As far back as the early 16Th century, "French pox" and "the French disease" were synonyms for genital herpes, and "French-sick" was another term for syphilis. The OED also equates the adjective French with "spiciness", as in "French letter" for "condom", French kiss (1923) and "French (i. e. "sexually explicit") novels" (from 1749)."
http://www.takeourword.com/Issue058.html
Compare the terms:
the French way (i.e., oral sex)
French postcards (pornographic pictures)
French lessons (prostitution)
See also:
http://www.phrases.org.uk/bulletin_board/13/messages/417.html
http://en.wikipedia.org/wiki/Pardon_my_French