Why is it "teach your girlfriend"? Why does he presume that his audience has a girlfriend who needs to be taught, rather than a boyfriend? You can't say "well he has a girlfriend so he wrote it like that" because he's not talking about him in the title, he's talking about you - he's presuming you have a girlfriend who needs to be taught, and that's sexist. I don't think it's the crime of the century, but I do think less of the guy.