Why does my boyfriend hide his female friends from me?

My boyfriend acts like he doesn't have any female friends and he doesn't care about girls like that. However, when I'm not around he hangs out with a lot of other girls. he's friendly to them, whatever. I even tried to introduce this girl to him and he acted as if he didn't care but later on I saw him hanging out without me. I saw him with her and he got embarrassed.
there's nothing wrong with having female friends but I don't see why he has to hide it.
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And after I caught him with her, he invited me to come hang with them.
Why does my boyfriend hide his female friends from me?
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