How is it that he is in a relationship with a girl and be flirtatious with another girl as if he is single?

I just don't get men like that. And yet be happy in his own relationship.

Why would he still kinda look around? And be sometimes on a dating app and then add them to his social media?

It didn't happen to me but I see it happening around me. So I'm curious why it happens.
How is it that he is in a relationship with a girl and be flirtatious with another girl as if he is single?
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