It's utterly grotesque and ultimately needless, their confrontation with IT as children in the books is completely philosophical for the most part and the reasoning for the sex scene is flimsy at best. He could have easily wrote it so that they find their way out of the sewers through the after effects of their battle with IT.
It's just... Ugh. It's something that completely taints the story for me. I pretty much have to pretend that chapter doesn't exist. It's just wrong.