Kedamono Dakara.

From Nakama: An employee at an excellent firm's real identity is actually as the family's ninja bodyguard. But his master is like a dim-witted princess. The "princess" always clings to him while he works and tells him "if you want to you can become my wife." He takes any chance he can to get close. The ninja always thinks "I'm not your sex slave!" but everytime he sees the "princess'" expression he is unable to resist and continues to sleep with him night after night. In order to separate the two of them, the higher ups have begun to quietly take action. Master and servant, this is a modern version of this kind of love story.