"The Mermaid problem is an observation occasionally mentioned in literature, concerning the difficulty of having sexual intercourse with a mermaid." - Jeanette Bosman

A French idiom, finir en queue de poisson (to end with the tail of a fish), makes reference to this difficulty; it refers to a promising start that ends in disappointment. - Private Sanjeev

Interestingly this was not always an issue. In the past it was not uncommon for a mermaid (actually a medieval siren or melusine) to be portrayed as having a split tail, with a vagina located (or merely implied to be) between the two parts. H. P. Lovecraft's short story "Dagon"[4] and the logo of the American coffee chain Starbucks are examples of this. In the original version of the logo the mermaid is shown spreading her tail apart up to her head. While this has been cropped out, and the drawing in general slightly reworked over the years, her tails are still visible around the edges. - Private Sanjeev