I think it was much more to do with the group having to come-of-age in order to defeat It, and that was the only way they could think of doing so.

As unpopular as it may be to say this, I dont think the scene in the sewers was really even about the sex. If you compare the way he writes about Bev having sex in the beginning of the book to how he writes Bev having sex with each of the boys in the sewers they are written VERY differently. The scenes with her as an adult are extremely sexualized and include actual violence, while the ones in the sewers dont describe the actual physical sex much as it does the way it binds her to each boy and how it changes them, as well as showing how it's more about the emotions than the physical act. Its awkward, none of them know what they're doing, and it's not about what they do but about what the act itself represents to them and how the milestone changes them, allowing them to fight and later leave the sewers.

It's a weird scene, and one I would never want to see in the film adaptations, but it does make sense in the book. I get what he was trying to do and how it was intended to show the coming-of-age, but it definitely threw me for a loop when I read it and I get why people aren't comfortable with it.