Does it have to do with hiding something or just invacion of privacy?

The other day I joked with my bf of 11 months and told him i secretly found his alo messenger password and found a girl writting to him...i told him i told her to back off....surprisingly he got upset and dint want to talk to me....i then wondered why he got upset after the fact that i told him i was joking...he took it the wrong way and then i asked him why was he getting so offended over something that has no meaning anymore....btw he has a lot of female girls on his aol messenger and claims he doesnt log on anymore since we hooked up as a serious couple....im a little worried because a little joke is giving me doubts....i need some tips and advice on why you guys think he got so offended all of a sudden.....