I've been meaning to ask Steve for years now to give his explanation for why the sex scene was included in IT. Better late than never as the saying goes, so here it is at long last in his own words:
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.