OP I'm the same with my boyfriend. When we first started going out it didn't bother me he'd slept with other girls, but gradually it started getting to me. It sounds strange but I can't explain why I felt like it. Looking back now, I think I was starting to get jealous...jealous that he'd been with other girls before me. But then it started getting to the point where I was wondering what they were like in bed compared to me, so I talked with my boyfriend and he assured me that it's only me he wants now. If he wanted them he would have stayed with them and not broken up, which of course makes sense. So maybe try talking to your boyfriend about it...I'm sure he will make you feel better on the situation, and reassure you that you are the one he is with now, not them, so you must be doing something right!!!
And don't worry about him being experienced and you're not...it makes it all the better for you!