Clark has to pose as an Evil Overlord of Earth to stop some invading aliens (or negotiate with other aliens or whatever *plot-handwave*) and any self-respecting Evil Overlord has a nice collared up pet. Bruce insists that he be the collared up pet because he couldn't possibly ask anyone else to do that, and also Bruce wants to be at the negotiations. Clark is uncomfortable and also turned on by naked, chained up Bruce kneeling at his feet.

Bonus: The invading aliens will ask why Clark's pet is so scarred and banged up, and Clark will explain to them that Bruce is a conquered prince of Earth, a great warrior, and that makes it a particularly great honour to own him because Kryptonians consider defeated warriors the most valuable pet slaves. The aliens insist on seeing a demonstration Bruce's fighting prowess. They are very impressed. Clark is very turned on.