Advice Needed!!

My boyfriend(20) and i(18) have been together for four years now. We have a pretty normal relationship where we argue every one in awhile but nothing too serious. Recently he just confessed to me after a joke i made about a former fling of his, who happens to be my friend, that he has had several fantasies about my friends and he often gets off on them. This made me uncomfortable so i questioned more and he said that he’s had a sexual fantasy about most of the girls in his life. I’ve always known he was more sexual than the average joe but i feel like this may be crossing the line. I asked him how he would feel if it was vice versa and it was me with his friends and he said he would also be uncomfortable but he just can’t control his sexual urges. He knows that this is weird and says he doesn’t like this trait about himself. I’ve done some research and a few have said this is normal and he’s just an idiot for telling me but when i asked my three male friends they all told me it was unusual and they don’t know how i’ll be able to get past it. Frankly i don’t either. so any advice or comments would be greatly appreciated. thank you!