Full honesty.

"Well, Uraraka, I'm actually a Sex Genie. This is basically happening via magic," Momo replied.

"...wait, genie? Magic? How?" Ochaco asked.

"I'm not going to go into the full specifics on how it's possible," Momo replied. "To be honest, calling it magic is seriously oversimplifying it. It still works well within the laws of the universe, just in principles way too over the heads of most people. But at any rate, basically, I got sucked into a lamp that came from another, distant corner of the multiverse and converted into a genie that can channel its power, at the behest of a Master. Specifically, I am a Sex Genie now. Before you ask, no, I don't mind it. Once liberated from internalized societal standards regarding sex, I really found it pretty appealing. The benefits way outweigh the drawbacks, especially seeing as I'm essentially now wired to enjoy the "drawbacks.""

"I... See..." Ochaco replied, her skepticism decreasing slightly. "And what exactly does this have to do with me?"

"Well, I'll let Master here explain that..." Momo grinned.

Izuku gulped slightly, before steeling himself, even moving his hands away from his manhood as he reminded himself that in this situation, there was no judgement. "...well... Momo and I are interested in expanding our relationship..." he said slowly.

"Expanding your relationship? Oh... So Momo wasn't just playing around when she told me I might be involved with you two in the future?" Ochaco asked.

"No... She wasn't. In fact, we'd both be very glad to have you with us... Are you interested?" Izuku asked.