Why would a male invite a female colleague out to lunch when his girlfriend happens to be there as well?

Anonymous
I was working with my boss's girlfriend yesterday as there's a lot to do for opening a new business & afterwards he made a quip about all three of us going for lunch. I accepted. We had some decent chats.

I am more curious why he invited me though... like why guys would you invite a woman to dine with you if you've your girlfriend. There was no reason for him to invite me really.
Why would a male invite a female colleague out to lunch when his girlfriend happens to be there as well?
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