To be as fair as possible, they're 11 rather than 9, and it's a train rather than an orgy.
That whole scene really is the culmination of a bunch of different things regarding the writing of It and Stephen King's career as a whole. King is not a creative writer when it comes to inventing new plot points and tropes, but he is was very good at using ones he already knew. The two major hallmarks of his writing was the willingness to spend pages to describe a scene in effective detail, and his willingness to use events in his stories that would be describe as vulgar or tasteless if less craft went into them. King also infamous for plotting as he writes, which has resulted in a reputation for endings that fizzle more than anything else. So you have a section of book, with the following real life and plot setup: 1) we're already 1000 pages in (and I will argue to the end of time that King initially intended for their to be a second, later encounter in the sewer that would be the real climax for the kids) which means it was already 200 pages longer than The Stand, which was maligned for being long; 2) King has a reputation for pushing the envelope for what is acceptable to portray in prose; 3) King has long developed a style wherein he describes a scene in explicit and not-quite-austistic detail; 4) King has been immune to editors for almost a decade by this point; 5) King is high as fuck on cocaine; 6) Of the Losers Club, Beverly has had little actual impact on the story (which is telling when Mike exists to be black, Stan exists to be jewish, and both have little impact on the plot in the kids section), and has been implied in the adult sections that she has had sex with the other members (though her character arc beforehand is the sexual attention she's starting to recieve from older boys and her own father, none of which are portrayed positively); 5) King at some point right before the end forgot that they had had no contact with each other for 26 years. The story outright states that their initial confrontation with Pennywise directly burned their metaphysical bond with each other which was instrumental to destroy It, and that the sewer scene was an act to restore that bond so they could finish Pennywise off in '85.
I understand, in this day and age where people will use their creative works live out their paraphilia (especially regarding pedophiles), that this kind of content can make one jumpy and morally outraged. I do not understand the modern puritanical stance from both sides of the aisle that the portrayal of an event in a piece of fiction, especially one of prose, necessitates advocating for that event. I'm sorry to dump this reddit-thesis here, but it's a criticism of King I've seen for a while now, from people who only have received their stance second-hand, when there's much more valid criticisms of King to be had.
