Mistress Bastienne felt that her husband took her for granted and she wanted to prove a point. She was both punishing him and she was also fighting for her marriage at the same time. So she turned him into a human toilet and she had him eat her shit and drink her pee. When she was done with him, she told him never to take her for granted again. He never did.
Mistress Bastienne is not the kind of girl you can mess up with and hope that she will not do something about it. This guy pissed her off and she had to strike back. She did it by inserting a butt plug in her asshole and then ordering him to lick it until it was clean. He had no choice but to do it and it was degrading and disgusting.