I don't really understand why people get so hung up on shit like this. It's disgusting and horrific or whatever but if it's important to the story or whatever I don't find anything wrong with it. It's not something I want to read but people get diddled and it informs the rest of their life. you can't always just slot in some other abuse for any other one without fundamentally changing the point. I read the book once as a teenager and I don't remember it too well but from my recollection I'd agree with your point that it's literally the hinge of the story having it be something else would make it a different book at best and not work at all narrative or thematically at worst.
Fuck's sake - I'm going to defend the pre-teen sex scene in "IT". FML
So the point of The Losers venturing into the sewers to confront Pennywise is that Pennywise already
knew their deepest fears and they'd have to overcome those to prevail, right? During the fight, Pennywise throws Stuttering Bill's dead little brother in his face, and Bill
overcomes that. And so does every other Loser when confronted by their greatest fear.
So they beat the spider and try to make their way back to the surface, but become hopelessly lost, and that's when Bev realizes
she is the only one who hasn't faced down her greatest fear and overcome it.
And yes,

, her greatest fear is sex. Daddy isn't the only adult who has (at least verbally) noticed her developing body and she's taken refuge against that amongst a bunch of boys who pretend to not think of her that way. So her offering sex to all of them in a dark sewer is her overcoming that last hurdle to getting them out onto the surface again. Frankly, I didn't find the scene overly graphic since it was written from her perspective. She's making a last, desperate sacrifice to get everyone out to safety. Gan understood that and provided guidance.
Is it still a bunch of pre-teens fuckin' in a sewer? Absolutely it is. Did it fuck up Bev in her later life? Absolutely it did. But did it get them back into clean air and sunshine so they could live to fight another day? It did.
King could have made her greatest fear be something else entirely, and that scene would not need to exist. Maybe she got scared and mentally-scarred by the monkeys at the zoo, for instance. Then all she has to do is bash in a fake monkey's head with a rock or whatever.
In other words: the pre-teen sewer fuckin' was a direct response to choices King made about Bev's character hundreds of pages prior. In the story, it's an appropriate response to those choices, but there's no reason that
had to be Bev's Greatest Fear.