Simon and Blue are emailing about whether or not Oreos are a food group. Simon insists they are, and tells Blue about an imaginary world he made up with Nora where everything is made up of Oreos. Blue responds and jokingly asks how many servings of Oreos he should have per day, and Simon responds with a detailed nutrition plan of Oreo products. He admits that thinking about this is making him hungry, and he used to fantasize about food as a kid until he learned about sex. In his reply, Blue says he can't eat deep-fried Oreos because he once rode a Tilt-A-Whirl after eating one and vomited profusely. He also says he likes imagining Simon fantasizing about sex.
Remember that Simon has already situated himself as a teen who's not necessarily interested in alcohol and hasn't had sex yet. This email exchange suggests again that these parts of Simon's identity are starting to change. In addition, this conversation also shows that contrary to what Simon thought of Blue, Blue is also thinking about sex and is therefore also coming of age and changing, even in the short amount of time that he and Simon have been emailing.