Why do women stay friends with ex-boyfriends?

DW-Diver
OK….maybe it is just me. I have been seeing a girl for almost a year. When we started dating she said she had male friends and that she was not going to change that, I was OK with that no issue. But now I find out almost all these male friends are former boyfriends (about 6 or 7 that I know of) that she has slept with. They are on her Facebook, here phone, she texts them talks to them, hell we even had dinner with one of them before I knew he was an ex. I am a very traditional conservative guy, is this normal these days? Am I just over reacting?
Why do women stay friends with ex-boyfriends?
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