If you haven't heard of "Mushroom syndrome" (I hadn't either), it's when someone is intentionally fed false or misleading information (i.e. horseshit) and kept in the dark. Similar to how mushrooms are cultivated. So the title is a play in that and also a tongue-in-cheek reference to the male anatomy. And it makes perfect sense here: the uke, Chuta, is a complete idiot, naive and way too trusting of the seme, Yu, who's his childhood friend. He knows nothing about sex so it's easy for him to get manipulated by Yu into being the target of Yu's perverted desires. He tells Chuta that guys have to do sexual things when their "mushrooms" get big so they can let the poison (semen) out or else it'll rot and they'll die. I can't... it's so ridiculous, but the idiot Chuta believes it and spends the rest of the manga in distress over "mushrooms" and "poison".
I think both characters are the same age but Yu has the size (and intellectual) advantage. He seriously comes off as a possessive, overprotective, creepy child molester, which is an apropos description since Chuta looks like a kid. All he does is grab, fondle and molest his dumb childhood friend in every chapter and in various locations where you'd think they'd get caught, like in a lecture and on the school bus! Typically, I wouldn't have a problem with this level of over the top dubcon but this was so stupid. One has to suspend disbelief in order to buy that Chuta, a HS student, is this naive. Also, Yu, despite being a complete sexual predator, has the gall to try to act morally superior in a scene where he saves Chuta. Zero self awareness which could've been funny but it wasn't. My eyes almost got stuck in my head from rolling them so hard.
And of course Chuta gets raped by Yu
...duh. That was a foregone conclusion. And of course one little ILY is all it takes to right the wrong.
I lost brain cells reading this hot garbage. It could've been so much better as a dubcon fetish smut, but it was just cringe because the bad writing and stupid characters. Plus, the art sucks.