Exactly how steamy is this story?
R
The Perks film was initially given an R-rating from the MPAA, and we'll second that in the book—just for the sex alone. There's not much steam in the book, but you'd be hard pressed to find a single virgin by the time Charlie mails his last letter.
Unfortunately, most of the sex that Charlie sees or experiences is damaging in some way. He witnesses a boy force himself on a girl, and most crucially, he finds out that he has been repressing memories of molestation by a relative. Even though Charlie doesn't devote much time to processing his molestation, the book makes it pretty clear that sexual abuse = bad and has some long-lasting psychological effects.
But what about sex between two consenting not-yet-adults?
We kind of get both sides of the coin. We see the consequences of consensual sex, like when Charlie's sister gets pregnant. After she tells her boyfriend the news, he denies he's the father, breaks up with her, and she has an abortion. Other times, the consequences of sex are unclear. Patrick has some pretty intimate moments with quite a few guys, and nothing seems to come of it. But even these encounters aren't glamorized; his promiscuity is just one of many "things to keep himself numb" (4.5.26).
While plenty of people have argued otherwise, it seems to us that the sex in Perks isn't gratuitous (i.e., it's not there just to shock us). While some sexual issues may be explored more than others, they're all there to make some sort of point. It's up to you to read the book and see just what those points are.