Some men do like to enjoy the flattery of flirting with a woman whom they know likes tikes them. They may even be amnoervering into a 'second string' situation, which is highly undesirable for you.
If he has a girlfriend and he is just being friendly, that's fine. If however he simply wants a little 'extra' shall we say, then don't demean yourself.
Put your energy into looking for someone you can have a civilised relationship with. Based on his perceived willingness to deceive his girlfriend, this man does not qualify for that role.