Tells him the situation and certain complications.

"Hmm... well, this actually makes the situation... tricky," Rias remarked, putting a hand to her chin as she pondered. "See, as a genie, I'm essentially your sex **** now."

Issei balked at that. "A ****?" he asked. He wanted a harem, but he wasn't out to subjugate anyone! He wanted things consensual!

Rias fixed her gaze on him. "That's almost always a bad thing, yes, so I understand your reaction... but there is more nuance to it than you might think. You hear of BDSM? Or, put another way, consensual submissive and dominant relationships? Although, that's technically not slavery, since by definition, slavery is against consent, but... well, there's something hot about the term, so let's use it anyway," she shrugged, making her boobs jiggle. "But, you see, the Lamp essentially turned me into a genie, granting me a higher state of being, at the price of being a servant of whoever owns the Lamp. Not much of a price though, considering the transformation comes with becoming extremely submissive."

"So... you're brainwashed into liking it?" Issei scratched his head.

Rias pouted. "I wouldn't say brainwashed, Master. More like... provided with an experience that is so enjoyable, you don't want it to end... albeit a bit forcibly, the entity that is the Lamp tends to ask for forgiveness instead of permission. But I stopped resisting the moment I realized just how good I was feeling," she purred, rubbing her hands over herself. "If I was "freed," I would just immediately try to become a genie again."

Relaxing slightly, Issei thought about all he knew about genies. "Okay, so what are the rules? Three wishes? More? And limits?"

Rias grinned. "Well, Master, Sex Genies like myself are limited in how we can use our new abilities. But not in the quanity. Normal genies can do almost anything, at the cost of only granting three wishes per Master. More specialized ones can grant as many as our owners desire while they have us, but at the cost of the wish having to be within our area of specialty. In short, as many as you like, but they have to be related to sex..."

"...not much of a limit," Issei replied, stars in his eyes. "You can use sex for just about anything, with just a bit of creativity... seriously, that's the basis for Rule 34!"

Rias giggled. "True... but now comes the complication. See, as King of the Peerage, I have you as my Pawn. At the same time, you own my Lamp... which makes me yours. Of course, your claim to ownership over me is stronger than mine to ownership over you, so it's up to you how you want to handle this..."