I have, myself, never really gave this scene a LOT of thought. Granted, I believe I first read "IT" in 7th grade? 1993/1994. I loved it; and was freaked out enormously. There are many scenes that WERE schocking to me: The boys at the dump, the switch blade to Henry Bowers dad's throat, most of the scene's involving Mike Hanlon's torment by Henry. The one scene I remember my friends and I being UTTERLY shocked by was the witch in Beverly's house. I had never read the word c**t in a book, and the witch talking to her, spewing that filth, I knew that it was bad (and being a 12 or 13 year old, loved every moment of "naughty" things). Re-examining the 80's, which I did not grow up in (Born in 1981), I think our society, for better or worse, has taken a step back from what is/was acceptable. I would say its a mixed bag; porn is everywhere, and far more shocking than that small part in the book. Do people remember the movie "Over the Edge", by Jonathan Kaplan? Or Christine F. (junkie 14 year old prostitutes in West Berlin)? I don't think that subject matter of those movies would be touched with a ten foot pole in this current era.
Now, is that good or bad? I can't say, honestly. I am not a cultural critic. Thinking back to when I did read the novel, both of the sex scenes stuck out like sore thumbs to me. Now, having two children and aging 24 years, I can understand (but not really buy the reasoning, and still feel repulsed by the scene itself) of the emotional aspect/connection Mr. King was trying to get across. Shortly after the sex scene, they also take a blood oath. Perhaps the sex scene was put in so Mr. King wasn't being redundant (rather silly, IMHO). The blood oath has always been more bonding in Mr. King's books; retrospectively, I think that should have been enough of a bond, especially with children protagonists.
Mr. King has gone on record as being a terrible editor of himself (aren't we all?). The Stand involves a man masturbating another man with a gun up his anus, a short description of a woman getting herself wiped in a private area with barbed wire, and (as a young person reading, this bugged me the most) a child falling down a well and dying of pain and starvation. And fancier, more Pedigreed writers such as Thomas Pynchon, have scenes of Corpophagia, etc.
So, in conclusion, I don't think any explanation would make sense in today's culture. Should it be in there? Yes, it's part of the story. Was it necessary? I don't think so, but hindsight is 20/20. I love the book, and don't think any less of it for either sex scene. Stephen King taught me to love literature, and I am eternally grateful to him for that. I think placing too much meaning on the "connection/bonding" aspect of the scene is wrong-headed. Is it wrong, in of itself? No. I never thought it was an important part of the story.
Sorry for going on so long! I am a terrible editor myself! I am trying my best to keep my expectations of the movie low, even though it is getting great reviews. I want to love it as much as I loved the book. Which is a tall order! haha.