Simon emails Blue and asks if he came out, and Blue says he didn't exactly come out. He explains that when he got to the hotel, Blue’s dad had everything arranged perfectly. Blue decided to wait until after they'd open gifts. However, he was shocked to receive History of My Life by Casanova, which he sees as evidence that his dad has no idea he's gay. Blue’s mom should be the first to know anyway, and Blue laments the difficulties of having divorced parents.
Giacomo Casanova (1725-98) was, and still is, known for his numerous sexual exploits with women. To Blue, receiving Casanova's autobiography likely comes across as a desire on his dad's part for his son to come of age sexually—specifically, with women. Blue's shock and decision to wait to come out to his dad illustrates just how destructive assumptions about other people’s identities can be.
Simon comforts Blue about having divorced parents and says he's planning on just sitting his parents down and telling them both when the time comes. Simon and Blue both mention how easy the other is to talk to, and Simon mentions that Mr. Wise says that Simon is bad about using sentence fragments. In his reply, Blue suggests that Simon is leaving more clues than he really should. Simon apologizes for the clue.
For Blue, the thought of learning Simon's true identity and merging it with what he knows of Simon through their emails represents a level of knowing that is, at this point, a little too much for him to handle. This is likely because Blue is still not fully comfortable with his own identity yet.