In a place such a Jikan, those rules are logical. But hearts are anything but logical. The weight of his gaze might seem heavy as Sesshomaru observes Zishu.

"I am not a toy." and so he will not be shared unless...

Both mates appeal to him. Sesshomaru has never been with a man and so he doesn't know how that would feel.

"I can see why she is with you."

Just as she had been alluring, so too is Zishu. And that is confusing.