The joke about Natsuki being followed by a hoard of guys seemed very strange for this manga. Up until this point the chapters have consistently framed Natsuki as an outsider. He's got friends as seen in chapter 11, but they're fellow outsiders. Most normies either keep their distance or gossip about him, as mentioned in chapter 3. Guys do hit on him at the start of each year, but immediately bail when they learn he isn't female, either from Natsuki directly or from "the rumors".
That was what made Fumiya different - he loves Natsuki knowing he's a guy. Much of Natsuki and Fumiya's chemistry comes from Natsuki cynically assuming that Fumiya doesn't really like him, and Fumiya proving him wrong. Whether Fumiya's relentless optimism will ultimately overcome Natsuki's sexuality is another question, but clearly Natsuki sees Fumiya as special. Quoting from chapter 3, "I guess how you're like this is why I don't mind being with you." A crowd of guys all shouting they're cool with Natsuki's gender isn't just unrealistic; it undermines what made Fumiya special.
Ultimately its a 1-panel joke that the author clearly didn't think too hard about, and so it doesn't need to be taken very seriously. But its kind of worrying, since it makes me wonder if the author really understands her own work. I guess we'll see if/when a 5th doujinshi comes out; I do still love this manga. I'm just a bit worried.