Why do men in relationships need their ego stroked by other women?

Anonymous
This question ( link ) was asked before but didn't get much response, so I figured I'd ask.

For guys that are in relationships, why do you feel the need to get attention/flirt with other women?

I've had issues in the past with my boyfriend doing this, and it nearly ended our relationship because he was being blatantly disrespectful. He said it's because of his insecurities and him just being stupid and not thinking.
Why do men in relationships need their ego stroked by other women?
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