Turning a slave man’s body into furniture is an excellent form of objectification. Sitting on him, using him as a chair or stool is a perfect reminder of his status as chattel. He is used as his female owner sees fit. His opinions and feelings do no matter.
Many male slave enjoy serving as a woman’s footstool.
If a slave is in need of punishment being tightly, uncomfortably bound under the weight of a woman’s body is very effective.