Okay, I'm gonna be pedantic, and know this is NOT me saying Stephen King was right to write this scene, BUT:

The kids did NOT defeat Pennywise by having an orgy. They had an orgy AFTER defeating the clown. They defeated the clown by not showing fear, which Pennywise feeds on. Not only that, in the book, one of the characters is connected to a giant cosmic space turtle (not joking) which helps them defeat Pennywise. Bev suggests having the orgy after because they knew they had to remember this experience as adults because Pennywise would come back, and all the adults in the town don't remember Pennywise. I can't remember the exact reason as to why that's the case, but that's the case. The act of sex was a way for them to connect childhood and adulthood so all of them would remember Pennywise in adulthood when he returned and needed to be killed.

Of course, there were a MILLION other ways King could've written this metaphor, but I understand what he was going for, even if it's gross and mostly unnecessary.