Correction not after they kill pennywise but after they fight him while they're children. They are still children when Beverly has sex with all of them. That's why it's so disturbing to people. I remember thinking why on earth is this even in the book?!
I remember now. I had forgotten. Weird that one would remember that scene in an 800-page book, full of weird sh1t (remember this is "horror" genre, where weird sh1t is normal, although it is the least favorite genre of conspiracy theorists) from start to finish (you must have liked, though, the giant tortoise ?? It has something to do with your "flat earth" right ?), not all of which is understandable anyway. I will remind you why it was in the book. Remember again, its horror genre, its Stephen King and LOT of illogical things happen in the book. This was a tiny part of that but apparently it was crucial in the children's SURVIVAL. Yes. They were stuck and desperate for a way-out. It happens in chapter 22, and by that time they already faced a lot of real-world and supernatural trouble. So desperate measures were par for the course by that time. For example Beverly had an abusive father who had an "eye" for his own daughter. Now THAT'S what is satanic, but you don't mention that. Each of them had their respective toxic trouble.
Its a bit long but help you get some perspective about the degree and extent of the "desperation". And desperate teenagers are highly innovative. At least this bunch was.
<Book Excerpt>
CHAPTER 22 The Ritual of Chüd
......
Bill was scared ... plenty scared. The conversation he’d had with his father in his father’s shop kept coming back to him.
There’s nine pounds of blueprints that just disappeared somewhere along the line.... My point is that nobody knows where all the damned sewers and drains go, or why. When they work, nobody cares. When they don’t, there’s three or four sad sacks from Derry Water who have to try and find out which pump went flooey or where the plug-up is.... It’s dark and smelly and there are rats. Those are all good reasons to stay out, but the best reason is that you could get lost. It’s happened before.
Sure it had. There was that bundle of bones and polished cotton they had passed on the way to Its lair, for instance.
Bill felt panic trying to rise and pushed it back. It went, but not easily. He could feel it back there, a live thing, struggling and twisting, trying to get out. Adding to it was the nagging unanswerable question of whether they had killed It or not. Richie said yes, Mike said yes, so did Eddie. But he hadn’t liked the frightened doubtful look on Bev’s face, or on Stan’s, as the light died and they crawled back through the small door, away from the susurating collapsing web.
“So what do we do now?” Stan asked. Bill heard the frightened, little-boy tremble in Stan’s voice and knew the question was aimed directly at him.
“Yeah,” Ben said. “What? Damn, I wish we had a flashlight ... or even a can ... candle.” Bill thought he heard a stifled sob in the second ellipsis. It frightened him more than anything else. Ben would have been astounded to know it, but Bill thought the fat boy tough and resourceful, steadier than Richie and less apt to cave in suddenly than Stan. If Ben was getting ready to crack,
they were on the edge of very bad trouble. It was not the skeleton of the Water Department guy to which Bill’s own mind kept returning but to Tom Sawyer and Becky Thatcher, lost in McDougal’s Cave. He would push the thought away and then it would come stealing back.
Something else was troubling him, but the concept was too large and too vague for his tired boy’s mind to grasp. Perhaps it was the very simplicity of the idea that made it elusive:
they were falling away from each other.
The bond that had held them all this long summer was dissolving. It had been faced and vanquished. It might be dead, as Richie and Eddie thought, or It might be wounded so badly It would sleep for a hundred years, or a thousand, or ten thousand. They had faced It, seen It with Its final mask laid aside, and It had been horrible enough—oh, for sure!—but once seen, Its physical form was not so bad and Its most potent weapon was taken away from It. They all had, after all, seen spiders before. They were alien and somehow crawlingly dreadful, and he supposed that none of them would ever be able to see another one
(
if we ever get out of this) without feeling a shudder of revulsion. But a spider was, after all, only a spider. Perhaps at the end, when the masks of horror were laid aside, there was nothing with which the human mind could not cope. That was a heartening thought. Anything except (
the deadlights) whatever had been out there, but perhaps even that unspeakable living light which crouched at the doorway to the macroverse was dead or dying. The deadlights, and the trip into the black to the place where they had been, was already growing hazy and hard to recall in his mind. And that wasn’t really the point.
The point, felt but not grasped, was simply that the fellowship was ending ... it was ending and they were still in the dark. That Other had, through their friendship, perhaps been able to make them something more than children. But they were becoming children again. Bill felt it as much as the others.
“What now, Bill?” Richie asked, finally saying it right out.
“I d-d-don’t nuh-nuh-know,” Bill said. His stutter was back, alive and well. He heard it, they heard it, and he stood in the dark, smelling the sodden aroma of their growing panic, wondering how long it would be before somebody—Stan, most likely it would be Stan—tore things wide open by saying:
Well, why don’t you know? You got us into this!
...
...
“I have an idea,” Beverly said quietly.
In the dark, Bill heard a sound he could not immediately place. A whispery little sound, but not scary. Then there was a more easily placed sound ... a zipper. What—? he thought, and then he realized what. She was undressing. For some reason, Beverly was undressing.
“What are you doing?” Richie asked, and his shocked voice cracked on the last word.
“I know something,” Beverly said in the dark, and to Bill her voice sounded older. “I know because my father told me.
I know how to bring us back together. And if we’re not together we’ll never get out.”
“What?” Ben asked, sounding bewildered and terrified. “What are you talking about?”
“Something that will bring us together forever. Something that will show—”
“Nuh-Nuh-No, B-B-Beverly!” Bill said,
suddenly understanding, understanding everything.
“—
that will show that I love you all,” Beverly said, “that you’re all my friends.”
“What’s she t—” Mike began.
Calmly, Beverly cut across his words. “Who’s first?” she asked.
...
...
...
Then she was in darkness, alone with the sound of the falling web and Eddie’s simple moveless weight. She didn’t want to let him go, didn’t want to let his face lie on the foul floor of this place. So she held his head in the crook of an arm that had gone mostly numb and brushed his hair away from his damp forehead. She thought of the birds ... that was something she supposed she had gotten from Stan. Poor Stan, who hadn’t been able to face this.
All of them ... I was their first love.
She tried to remember it—it was something good to think about in all this darkness, where you couldn’t place the sounds. It made her feel less alone. At first it wouldn’t come; the image of the birds intervened -
...
Yes, the birds, I was thinking of them because I was ashamed. It was my father who made me ashamed, I guess, and maybe that was Its doing, too. Maybe.
The memory came—the memory behind the birds—but it was vague and disconnected. Perhaps this one always would be. She had—
Her thoughts broke off as she realized that Eddie
comes to her first, because he is the most frightened. He comes to her not as her friend of that summer, or as her brief lover now, but the way he would have come to his mother only three or four years ago, to be comforted; he doesn’t draw back from her smooth nakedness and at first she doubts if he even feels it. He is trembling, and although she holds him the darkness is so perfect that even this close she cannot see him; except for the rough cast he might as well be a phantom.
“What do you want?” he asks her.
“You have to put your thing in me, ” she says.
He tries to pull back but she holds him and he subsides against her. She has heard someone—Ben, she thinks—draw in his breath.
“Bevvie, I can’t do that. I don’t know how—”
“I think it’s easy. But you’ll have to get undressed.” She thinks about the intricacies of managing cast and shirt, first somehow separating and then rejoining them, and amends, “Your pants, anyway.”
“No, I can’t!” But she thinks part of him can, and wants to, because his trembling has stopped and she feels something small and hard which presses against the right side of her belly.
“You can,” she says, and pulls him down. The surface beneath her bare back and legs is firm, clayey, dry. The distant thunder of the water is drowsy, soothing. She reaches for him. There’s a moment when her father’s face intervenes, harsh and forbidding (I want to see if you’re intact)
.....
She thinks of birds; in particular of the grackles and starlings and crows that come back in the spring, and her hands go to his belt and loosen it, and he says again that he can’t do that; she tells him that he can, she knows he can, and what she feels is not shame or fear now but a kind of triumph.
“Where?” he says, and that hard thing pushes urgently against her inner thigh.
“Here,” she says.
“Bevvie, I’ll fall on you!” he says, and she hears his breath start to whistle painfully.
“I think that’s sort of the idea, ” she tells him and holds him gently and guides him. He pushes forward too fast and there is pain.
Ssssss!—she draws her breath in, her teeth biting at her lower lip and thinks of the birds again, the spring birds, lining the roofpeaks of houses, taking wing all at once under low March clouds.
“Beverly?” he says uncertainly. “Are you okay?”
“Go slower,” she says. “It’ll be easier for you to breathe.” He does move more slowly, and after awhile his breathing speeds up but she understands this is not because there is anything wrong with him.
....
Did you?”
“Did I what?”
“Whatever it is. I don’t know, exactly.”
......
Mike comes to her, then Richie, and the act is repeated. Now she feels some pleasure, dim heat in her childish unmatured sex, and she closes her eyes as Stan comes to her and she thinks of the birds, spring and the birds, and...
...
There is a long wait, and then Ben comes to her.
He is trembling all over, but it is not the fearful trembling she felt in Stan.
“Beverly, I can’t,” he says in a tone which purports to be reasonable and is anything but.
“You can too. I can feel it.”
She sure can. There’s more of this hardness; more of him. She can feel it below the gentle push of his belly.
...
You laugh because what’s fearful and unknown is also what’s funny, you laugh the way a small child will sometimes laugh and cry at the same time when a capering circus clown approaches, knowing it is supposed to be funny ... but it is also unknown, full of the unknown’s eternal power.
...
<End of excerpt>
And thus , they find their way out of the tunnel-maze. That sex(s) was some kind of spiritual ritual that pre-teen Beverly knew instinctually. It was not underage slutty behavior. Yes it was weird looking at it isolatedly, but I remember NOT feeling weird reading it. Because whole book is weird, and this scene gelled well with the story.