Martin writes Simon an apology email. He says he can't stop thinking about what Simon said about taking something personal from him, and he understands it now. Martin says he took the Tumblr post down and admits he feels sick with guilt about everything he did. He starts to explain why he did it.
Martin's email is a true apology—he takes ownership of his actions and actually apologizes. This is one way for Martin to show Simon that he does care about him and genuinely regrets what he did.
Martin starts by saying he's not homophobic and thinks gay people are awesome. His brother came out over the summer, and it was a big deal for Martin's family. His parents now march in Pride parades, but his brother doesn't even seem to care. Martin felt weird about all that, and also admits that he felt desperate about liking Abby. He admits that he's jealous that Abby befriended Simon over anyone else. Martin apologizes again, wishes Simon good luck with Bram, and says he enjoyed getting to know him. He closes by admitting that if he could do it again, he'd only blackmail Simon into being his friend.
When Martin brings up his mixed feelings about his brother, it tells Simon that his family isn't the only one going through major changes and struggling to adapt. When put in this context, it makes more sense why Martin felt so desperate and lost and felt driven to blackmail. Essentially, this makes it clear that familial strife and change is extremely difficult for everyone, and not all families can successfully change.