An altogether more accurate term for 'bum sex', as there are actually willies involved in it, as these authors well know. It also undergoes frequent use as a rudey exclamation of horror, when something very bad or irritating happens.
'Rachel likes to have willy bum sex with boys.'
'Kate has willy bum sex five times a day.'
'OH WILLY BUM SEX, i forgot the biscuits :('