Well, it has been mentioned that pornography is one of the main reasons people break up, married or otherwise, and erotica is a medium of pornography.
She may be playing it off as teasing him, but I'm positively certain she's still mad at him and is giving him a hard time.
And as for 'It's a boyfriend's job to avoid doing things that upset his girlfriend' quote ... not only is the reverse also true, but there are just some things in this world where such an outcome is unavoidable. There are girls who will get upset over guys enjoying their innocent and harmless hobbies and will break their souls over it, which will also lead to a break up.
As cool as MC was unintentionally making himself out to be, 'making your romantic partner upset' is not a matter of 'if', but 'when', and how would the two of them work it out once it's out in the open is the key to making their relationship stronger, or break off because of it.