It is all about your comfort level with her in terms of trust and in terms of her personality.
One of the guys I love most dearly in the world as a friend is an incalculable flirt. It is just his way. He would never cheat on his wife. They have been married forever, but he flirts constantly with everything female. When he flirts, he is very physical. He kisses me on the mouth (which I usually do not let anyone except my husband do), stands with his arm around me, etc. Like I said, he is this way with everyone.
As much as I love the guy as a friend, as much as I admire his character, I have often thought to myself, “God bless, Ann, I could not be married to him.” She just doesn’t seem to be bothered by it at all. I would go crazy.
It is not about not trusting my husband. I do. I am just not comfortable with that kind of interaction from either of us in a committed relationship.
So, what you need to think about is this: If this is your girlfriend’s personality, it is unlikely to change now or change should you two become life partners. You then have to ask yourself if you can make peace with that being who she is. If the answer is no, then you need to break up with her. Otherwise, your life will be a contant battle, and you will be constantly unhappy.
Have you had an open conversation with her about your discomfort? Not her behavior, because she is doing nothing wrong, but your discomfort? She does not notice the guys flirting? That likely means she is not interested.
She has chosen you. If you trust her, remember that.
I would not confront the guys. You can do other things. Walk up, put your arm around her, and drop a kiss lightly on her, then say, “Want to dance honey?” Look at the guy and say, “Sorry, I am going to sweep my lady off her feet.”
Or Want to get something to eat, honey, Want to check out the deck, fireplace, patio, etc.
Note: You will not be able to carry this off if you cannot do it in a charming manner and a casual, lighthearted way.