Is it normal for my boyfriend to picture other people when we have sex

My boyfriend and I have been dating for almost a year now. He recently told me that he's pictured other people when we hook up, quite a bit. After a few months of hooking up he was having problems staying hard and finishing and he said he started thinking about other people then so he could finish. Anyway, hes done it so much now that its become a habit for him and more often than not he is picturing other people when we hook up. I don't know if its normal for guys to do that or if it shows lack of interest in me? He said he primarily pictures a girl he had liked before me for 3 years and 2 of my best friends although there were others... I always thought we had a connection and he says there is and that he loves me but if he is thinking about and finishing to others while im all about us and what were doing does that mean the connections all in my head? Is it that he is just not sexually attracted to me so he has to picture others to finish? I guess my main question is this: Do most guys picture other girls when they are having sex with the person they want to be with? And is it concerning thats it mostly the same people that we both actually know and not celebrities or anything?