I finished reading the book, yet again, on Wednesday. I have to say that I really had no problems with this scene. For the first time I felt it really had nothing to do with sex, but more to do with the strengthening of their friendship so they could escape the tunnels beneath Derry. They were becoming just children again, and losing that special bond they had been given. Remember, they would never again be all together after this. They would never meet and play as 7 again. Because of the lose of this bond, panic was beginning to overtake them, Eddie could no longer guide them with any hope of escape. Their only chance was to try and recreate that special bond, they had to come together as one if they had any chance of surviving.
Beverly recreated that bond in the only possible way. It wasn't about sex, but love and friendship. The act brought them back together just long enough to escape the dark tunnels.
Even the description that Stephen gives us is not as bad as I thought I remembered it.