It does sound unreasonable on his part. Most chaps like their girlfriends to make little mistakes, especially about the offside rule, so that they can have the pleasure of correcting them. Besides, it's almost obligatory to spell things oddly in an SMS.
Maybe you should have pointed out that he should have written "YOU HAVE MADE A MISTAKE AND IT'S A MAJOR TURN OFF FOR ME".
Anyway, send him over here, and I'll slap him for you.
Have a cheerier Thursday,
MrP