Lady Cary felt that it was important for her to do what she wanted and to achieve all that she had sought to. The mistress did not want anything to get in the way of her and her fun and that is why she chose to use her hot ass to turn the guy on. It worked out the way she wanted and it was a lot of fun for her.
Lady Cary did not trust her husband to behave well when she was gone. She was about to travel out of town for a few weeks and she wanted to make sure that he would not misbehave while she was gone. That us why she had to make sure he wore a chastity device whose key she had. The only thing he could not do without her was to pee.