Exactly how steamy is this story?
PG-13
Although Karou and Akiva steam up a few pages of Daughter and Smoke and Bone, that's all a prelude to an act that never happens. Sure, Karou talks about losing her virginity to Kazimir, but that all happened off-page as well.
And while Brimstone lectures Karou about not succumbing to the temptation of "inessential penises" (3.34), this talk is about as sexy as discussing the birds and the bees with your dad. Finally, there is a bit of full-frontal male nudity, but it's done tastefully in an academic setting.
Think of the iconic drawing scene in Titantic, but with Zac Efron standing in for Kate Winslet, and you'll see just how steamy PG-13 can be.