Someone actually explained this above, and it was explained to me as thus. To face IT, only a child could enter his realm. Even later in the books when they tried to ed IT once and for all, they had to become basically "childish" all over again. At the time of the sex scene, to escape from IT's clutches, they needed to stop being children. They had to be adults. Look at our culture, and the vernacular used. When a male has sex, it's referred to as the woman "making him into a man!" It is the single act that is universally seen as the act that changes one from a child to an adult in every single culture. So the sex, while seemingly hella out of place, is actually the only thing a child would likely think of to become an "adult", especially after Bev's father cut her down and used this possibility as a weapon to hurt her.
If you look at this as a "Rite of Passage" in order for them to escape the influence of IT, it makes total sense.
If you look at this as a "Rite of Passage" in order for them to escape the influence of IT, it makes total sense.