Warning: SPOILERS for One-Punch Man Ch. 171The climactic battle of One-Punch Man's Monster Association arc led to a lot of casualties, but as it turns out, surprisingly few of them were fatal. Even for the monsters, it seems that several of them survived in various ways, including the series' weirdest monster of all.
While the S-class heroes ended the battle suffering from significant radiation poisoning, none of them actually died--at least in the new timeline that Saitama accidentally created, anyway. And among the villains, it was already shown that Psykos had survived her battle with Tatsumaki, and Garou survived with only the loss of his divine power. Other monsters, like Fuehrer Ugly, seemed to certainly be dead, but a few were left unaccounted for, and chief among them was Black Sperm. During the battle, One-Punch Man's Black Sperm had created 54 trillion bodies, so hiding some away for survival in the event of a loss was a distinct possibility.
When Black Sperm first fused into his final form, Platinum Sperm, he brought together all but 100 of those bodies, which immediately attempted to flee. Metal Bat then killed at least one of those, but the remainder went unseen. Chapter 171, by Japanese chapter count, reveals that S-class hero Pig God had returned to the battlefield in search of surviving monsters--and was completely right to do so, as he stumbled across the remnants of Evil Natural Water, which managed to survive Saitama's Serious Punch. Black Sperm's apparently lone surviving body was nearby as well, cowering under a rock, when he witnessed Pig God devour Evil Natural Water and digest it, finally killing the monster for good. Seemingly satisfied, Pig God then left, causing a terrified Black Sperm to decide his best bet for survival was to befriend someone powerful--and then he spotted Saitama and Genos with the monster Overgrown Pochi, who'd also shrunk down to regular dog size after defeat.
While the chapter ends before Saitama's reaction is given, it's not exactly in character for him to attack someone out of nowhere, even if they're a monster. Genos might, especially since he witnessed Black Sperm's actions during the battle, which Saitama was not present for. Still, Black Sperm poses no threat right now, and if Saitama was able to best Garou, he can certainly defeat Platinum Sperm should it come to that. It's already been shown with One-Punch monsters like Oculette that being a monster doesn't inherently make one evil, so there's always a chance for redemption. At the very least, keeping Black Sperm nearby is an easy way for Genos to keep an eye on him and potentially destroy him should he make a turn towards evil again.
Those who haven't read One-Punch Man's webcomic may be surprised that of all the monsters who could've befriended Saitama, Black Sperm is the one to actually do it. Having a sneaky monster hanging around with the heroes could add a bit of fun to One-Punch Man, much like the character of Dimple in ONE's other series, Mob Psycho 100.