Why would a guy flirt with another girl on our date?

We’ve known each other for over 2 years, but just recently started going out. On our first date he told me his intentions were to pursue something serious in the future. Nonetheless, last time we went out with my friends, he started flirting with another girl right in front of me. He even offered her a ride home and to join us on our table, yet never introduced her to any of us.

Why would a guy flirt with another girl on our date?
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