There are a lot of theories on that scene. One of them is that they were looking for comfort after their horrific encounter and the girl was the only one to provide it to a bunch of boys. My favorite theory is this one; everyone lost their innoncence that day when they first encountered IT. So from a child they unwillingly entered adulthood. The sex-scene was sort of a 'rite of passage' after a traumatic experience to mark that transition. Somehow of a reminder that their childhood was over and this was 'what grown-ups do'.

Then again, King is a goddamn pervert in most of his books. Some scenes are totally unnecessary and are mostly shock value. I think he derives some pleasure out of the fact that people are reading those scenes and get upset about them. Nevertheless, great author!