There was an entire Japanese movie I once remember watching about that. This man and this woman got erotically obsessed with each other after having sex when either one or both of them had been about to go to the toilet, because they were suddenly so randy for each other the bladder just had to wait. Following that they associated a full bladder with sexual arousal so it would turn them on.
I don't remember what it was called; just that the couple had random sex in various locations, often outdoors (they apparently also had a fetish for potentially being caught at it), and in the end when they can't milk any more novel excitement out of their combined fetishes, decide to just end it all because apparently they have no other meaning in life outside of their bizarre affair and chasing new sexual highs. In the final scene she's curled up next to him after an overdose of pills, having cut off his penis (at his request) and written some Japanese characters on his body with the bloody end.
It was a morbidly fascinating watch.
