Boyfriend's sexual past?

So I recently discovered my boyfriend's lengthy sexual past. Apparently he has slept with somewhere in the upper 20's. This includes multiple of his friends current girlfriends or ex's of friends, girls I know of or was friends with, etc. He had done all this throughout a 4 year relationship with his only real girlfriend. He was up front and honest and told me he cheated on her several times. Today a girl I knew of added me on Instagram but I wasn't sure how I knew her, so I asked if her name sounded familiar and he said "yeah I slept with her" ( I would rather him tell me the truth than lie about it to protect my feelings). The problem is that I feel like every time I turn around, I stumble on another girl that he's been with and it honestly makes me feel a little embarrassed that so many girls know my boyfriend that way. Am I overreacting? I know all of this was before me and I know that being with someone is about accepting who they are, including shitty pasts, but for some reason it tends to bother me when he admits to another one. Thoughts?