Delta: The Tower

This is an adult book, only for sale to people over the age of 18. Elkie and Gill like each other, men and public urination, not necessarily in that order. At the start of this short story Elkie stands by the railing at the top of a high tower and pees over the edge; far below, and unseen by her, Ian and Greg get an unexpected shower from a cloudless sky. Later the two couples meet up and find they have a lot in common.
For the next hour or two they enjoy an orgy of sex, peeing and pee-drinking, with some mud-wrestling thrown in. Do not buy this book if you might be offended by graphic descriptions of such scenes.