Well shit, come clean and hope for the best.

"I was kinda hoping you would ask..."

"No mistress Hu?" She looks you up and down "sounds like you're an illegally free male"

"N-no! I've got my paperwork!"

"If I remember my law work correctly that paperwork says you have to accept the first offer of a mistress you receive right?"

"...yes."

"Well problem solved! I'll be your mistress!"