There's not a lot of sex in this book. But that doesn't mean there aren't hints of things, if you read between the lines. Take, for example, the relationship between Isabella and Captain Tilney. Their dialogue is downright scandalous for this period, and Catherine is mega-embarrassed when she overhears them. It's doubtful that Isabella and the Captain were playing chess together when they were alone. Or that James dumped Isabella because she was chatting up the Captain, who totally has a reputation for seducing women.
Also, those Gothic novels that everyone in the book loves to read were often pretty scandalous and some had way more overt hints at sex than Austen's text. So Catherine may have been reading about some pretty risqué activities.