Toilet training as an aspect of obedience training can be sexy....Its not the "act" of urination that is sexy but rather the aspect of control...Unless you're into humiliation play, then its sexy for that reason too I guess.
Toilet time is socially thought of as being "private" so when someone can go past that in order to follow a command from an Owner. that challenges ones social reasoning...and bubble of comfort....Totally fucking sexy.