In an email to Blue, Simon refers to middle school as an "endless horror show" and lists some events that helped him discover he was gay, such as a weird dream and his middle school obsession with the lead singer of the band Passion Pit. He then recounts his "finest moment" of having a girlfriend in eighth grade and going to the dance with her. After spending the dance under the bleachers with his friends, Simon was terrified when someone told him that his girlfriend was waiting for him outside. Rather than meet her to make out, Simon hid in the bathroom for the rest of the evening. He says that even though he knew he was gay then, he had two more girlfriends after that.
Since middle school encompasses some of the early stages of puberty, middle schoolers are just beginning to discover who they are, and sexuality is a large part of identity. Simon's levity (describing middle school as an "endless horror show" and clearly dramatizing events) shows that Simon likes to perform for people and dramatize his life, which can in some cases play into him performing identities that he thinks people want to see, rather than what's authentic to him.
Blue writes back that it would be depressing if Simon's actual proudest moment took place in middle school, as middle school was absolutely awful for everyone. Blue then explains that he's seen the man who instigated his own discovery of his sexuality, Mr. Sexual Awakening, several times, though he's straight and his wife is pregnant. Blue closes by asking why Simon had girlfriends if he knew he was gay. Simon writes back that he didn't fully believe he was gay.
Though the tone of Blue's question about Simon's subsequent girlfriends isn't clear (either to Simon or to the reader), it possibly suggests that having girlfriends doesn't represent a "right" way to be gay according to Blue. This introduces the idea that the process of grappling with one’s sexuality and figuring out their identity is different for everyone.