Why doesn't her boyfriend want to put pics of her up on his Facebook?

My sister's boyfriend tells her that he doesn't want everyone in their business and that's why he doesn't put pics of them up together on his Facebook. However he had pics of him and ex girlfriends up and in his photos yet he doesn't have not one picture of my sister in his photos or anything! She comes to me about and thinks he's ashamed of her so he told her to just stay off fb and he would stay off also. Why doesn't he want the world to know about them or even family and friends for that matter? I mean everyone is on facebook and people in relationships post pics of them out on holidays and everything. And also he told her don't put pics of him on her Facebook.... She fills he's ashamed of her or hiding her what do you all think?