Exactly how steamy is this story?
PG-13
Sex is of frequent concern here in The Once and Future King. Let's face it: one of the central plot lines involves an affair between a married woman (Guen) and her husband's bestie (Lance). Aside from that, there's that whole accidental incest situation with Arthur and Morgause—as well as some references to deflowering virgins, rape, and a brief mention of homosexuality. (Galahad is described by Gawain as a "catamite," a word which is not in most modern people's working vocabs but means "homosexual.")
But let's just wait a sec before we break out the black censoring boxes a la network television. Even though there are many references to sexual situations, these are usually brief, matter-of-fact, and definitely not graphic.
For example, the deed itself is usually referred to by handy-dandy circumlocution: "sleeping with," or "making love," or "going to the Queen's bedroom" (maybe that one will catch on as a euphemism?). So, there are really no details to be found.
Pretty much the most graphic description White gives us is of the tapestry of Bathsheba in her bath that decorates Arthur's Justice Room. She's sitting in the tub "with her two round breasts" (C.8.1). That's as far as White goes, though.
All in all, White's quartet is pretty tame—especially for students accustomed to A Game of Thrones.