The application of urine as a form of sexual activity. Urolagnia is considered pretty taboo, and in the UK, the BBFC refuses point black to pass any film, pornographic or not, involving this scene, not even at "R18" (equiv. of "XXX" rating in US).
BBFC report on the thought-provoking chef d'oeuvre "Liquid Love" -
"When submitted to the BBFC the work had a running time of 239m 35s.
To obtain this category cuts of 121m 0s were required., some or all of these cuts were substitutions. The cuts were Compulsory.
Cuts were required to remove scenes of urolagnia in accordance with the current interpretation of the Obscene Publications Act and to remove sight of abusive and degrading activity (gagging during deep throat fellatio) in accordance with current BBFC classification Guidelines and Policy. R18 sex works may only be sold in a licensed sex shop."