Why do men that have girl friends flirt with other woman?

I'm writing a paper and need some opinions. For example there are men who will take the chance to get to know a person when they already have a girl friend and tell them how much they like talking to them. This usually leads the opposite sex to think that the male is interestedWhy do men that have girl friends flirt with other woman?
it boosts their ego. they may really care about their girl friend. males just like the feeling of winning or accomplishment and when they can get a girl to flirt back, they get that feeling that they love.Why do men that have girl friends flirt with other woman?
They have no respect for their girlfriend.
They want to know that they are still attractive to other people.
Well, Males are dominant figures, they like to have power and be in control. Throughout history and society Males show powerful status through money and women. It's pretty ';innate'; for a male to want to be desired by woman. The more females that like him the more confident and domiant he will feel.
Because they are men. We can be flirts. Just like women.