First, he explains how he knows.

"Ah, five minutes early! I do so appreciate punctuality!" Nezu remarked as Izuku and Momo entered, Ochaco waiting outside for them.

Both genie and Master took their seats, and gazed at Nezu curiously, and glanced at the seat next to them, which held a rather apologetic looking All Might in his normal, skinny form. "...so, I probably already know the answer to this, but what do you know?" Izuku asked the diminutive principal.

Nezu chuckled. "Everything about Momo being something of a sex genie. Her personality didn't fully match her profile, you see, though I initially chalked it up to her simply having a boyfriend," he shrugged. "But then there were some strange occurrences, especially when Aizawa informed me about how you suddenly seemed to gain better control of your Quirk after having a whispered conversation with her. Nothing like the damage you caused to yourself during the entrance exam!"

"...and that was enough for you to piece things together?" Momo asked skeptically as she decided to get up, and sit in Izuku's lap instead of her own chair, seeing no point to hiding affectionate gestures from Nezu at this point.

Nezu laughed a bit at that question. "Hardly~! I'm hyper intelligent, not omniscient! It was merely enough to gain my curiosity is all! Then, when I saw you and your other girlfriend go to All Might's office together via security camera footage, I asked him what you all talked about? I may have bluffed a good amount and made some insinuations that I already knew, and merely wanted to hear it from him!" Nezu chortled, and All Might blushed and looked away in embarrassment for having fallen for such a basic trick. In his defense, it was basically impossible to tell what he had on you and how he got it unless he outright told you.

"...I see," Izuku said carefully as he wrapped his arms around Momo. "...and you're not going to do anything about it?"

"Why would I?" Nezu waved Izuku off. "So long as you continue not to cause any harm, I have no reason to intervene. I'm not going to judge you for getting a willing harem either. So long as the key word remains "willing."" Nezu sipped his tea, and then replied to them, "I really don't have any of those silly standards you humans seem to have when it comes to sexual expression, so long as it's between consenting adults. I am an animal, after all. Really, the only reason I even bother wearing a nice suit instead of just letting my fur cover me is so people take me more seriously."

"...I see..." Izuku sighed in relief. He had known from the previous wish that Nezu was at least okay with it all, if not approving, then at the very least not disapproving, but it was reassuring to have it confirmed regardless.

"So then, onto important matters!" Nezu replied, clapping his paws together.