Exactly how steamy is this story?
PG-13
The romance between Po and Katsa offers a fair amount of steam as they flirt back and forth, even though much of the flirting is unintentional on Katsa's part in the beginning. She keeps finding her eyes drawn to the neck of Po's shirt, whether it's open or closed, and can't keep herself from commenting on it or going red in the face whenever he makes a suggestive remark.
When the two of them finally hit the road, she has a hard time keeping her mind from wandering into the land of lust when Po takes off his shirt and sits in front of the fire, and we feel like we know then and there where this is headed. And that's exactly where it goes. Katsa and Po become lovers, and the first time they are together, we do get a fair amount of description of mouths seeking mouths and hands fumbling at clothing.
So yeah, there is sex, and there is some description of it, but it's not exactly what we'd call graphic. It's more like in a movie, when you see two people starting to get down and then the scene fades out and comes back with them lying together under the covers, snuggling and chatting. You know what's going on, but most of it is left to the imagination, which keeps this book in the PG-13 range.