Sex with his ex

My long term boyfriend has had sex with two of his past girlfriends. I can't get the thought of him having sex with other people out of my head. Both of them were a long time ago but it makes me sick to my stomach to think that he has been intimate with other people. I know it shouldn't bother me but it does. We've talked about it a little bit but it still really bothers me. Any advice on how to get past it/over it?