Lady Scarlet is a clean freak and that is why she did not like it when she noticed that her slave did not observe high standards of cleanliness. She had to do something about it and she used her pee to punish and to humiliate him. Lady Scarlet put a funnel in his mouth and she used it to pee into his mouth. He was shocked but he quickly learned his lesson and that is how he avoided further degradation.
Lady Scarlet wanted to try something new for foreplay and that is why she chose to use her pee to do it. The mistress knew her boyfriend had never experienced that but since he loved how naughty she was and he wanted to experience new things all the time, she chose to surprise him with her pee fetish during foreplay without giving him any warning. She was shocked that he did not complain.
Lady Scarlet wanted to show this loser that she was not going to accept anything short of him doing exactly what she asked. That is why she used her pee to dominate him and to teach him a lesson. The mistress made sure she peed into his mouth and he had to choice but to drink it. He tried to refuse but she told him he would eat shit instead. He drank the pee.