Sordidnyms are phrases of seemingly innocent words which, when put together, and exposed to a dirty mind, become sordid in their implication. Additionally, it is a convenient way to mask the author's lewd tongue.
They are used where euphemism would generally be employed.
Spank the monkey, Feed the pony, and Toad the wet sprocket (the latter courtesy of Monty Python) are better known examples of sordidnyms.
Other creative samples are: