It's only partly revenge. Gregor does not give into the impulse to get his own in without another purpose. It's also a declaration.

"I want a point made that this goes both ways," he corrects. "If you find a way to subtly imply this, feel free. We are not blindly playing out tradition. You are loyal to me but you are not servile." Gregor is not a tyrant making use of what is his. Which surely he can't argue with, as it should be better for his reputation. There are several possible flavors of this arrangement and if picking this one with the Emperor is slightly scandalous, well, that suits him. Gregor has never done what anyone wanted of him romantically or sexually anyway.