Fwiw, this is the only statement I could find from King regarding the scene:

I wasn’t really thinking of the sexual aspect of it. The book dealt with childhood and adulthood –1958 and Grown Ups. The grown ups don’t remember their childhood. None of us remember what we did as children–we think we do, but we don’t remember it as it really happened. Intuitively, the Losers knew they had to be together again. The sexual act connected childhood and adulthood. It’s another version of the glass tunnel that connects the children’s library and the adult library. Times have changed since I wrote that scene and there is now more sensitivity to those issues.

Basically “Idk, it seemed like a good idea at the time, lol”

I can’t speak to how turned on he was when writing it - but as others have noted, his writing style was “do lots of drugs and then mail it to the publisher.” So its at least possible that he is being completely forthright above. Honestly, the weirdest thing to me is that the publisher didn’t nix it.