It is kind of gross, but so are many things in many books, including in that one (eg all of the disembowelings)

For me it is less problematic than it could have been, because: YT is shown to be naive, reckless and impulsive, and Raven (is that his name? I think so) is an awful person just in general. The author is in no way implying that sex for those two is a good idea, in any way. It's just a thing that happened in the book.

It's also not a sexy scene, so it's not written to be titilating (IMO), and it's short and it does ~something for the plot.