Guys, be honest why do you flirt with other girls when you are supposedly seeing someone else?

I know this guy that flirts with me when his girl isn’t around, he is touchy feely but then he brings her around sometimes. The other day when we were all at the same place, when she was talking to other friends he came up to where I was but I totally ignored him and didn’t give any contact and he eventually walked off. I’m sure he hasn’t told her how he flirts with me at times. If he really likes her why does he flirt with me? I just don’t understand why guys flirt when they already have someone

Guys, be honest why do you flirt with other girls when you are supposedly seeing someone else?
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