Why do some women that have boyfriends flirt with other guys?

I smiled at this woman because I think she’s attractive and hey why not. Anyway she smiled back and twirled her hair. Now I didn’t know she had a boyfriend until my friend told me. He doesn’t feature on her Instagram either so I assumed she is single. Sure she doesn’t have to advertise she has a boyfriend but why do some women flirt with guys when they’re in a relationship? How come? Like if I was her boyfriend I wouldn’t flirt with other women but why does she do this? Please let me know what you think

She’s bored
She likes to be cheeky
Sometimes people meet new people
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Why do some women that have boyfriends flirt with other guys?
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