What type of girl do you have?
I see it this way:
If you have a girlfriend and some guy is hitting on her and is making it clear that he wants her, then in order to avoid temptation (we ARE all human and as strong as we think we are, we are often tempted into things.. some people take short, some longer, but the temptation is always there). Anyways, if he is making it clear that he wants to get sexual with her / wants something more than just her friendship, then she should not have to even be told what is right, she should naturally tell him that she is not interested and try and avoid him.
If your girl is an attention wh0re, then she will probably enjoy this other guys attention and go with it. Most immature women love attention and will get as much of it as possible. And if your chick is that type of girl, then I would recommend preparing yourself for heartache and to find someone else.
Guys are always going to flirt with your girl, especially if she is hot, there is nothing you will ever be able to do to stop that. It is how your girl reacts to these guys that matters and will show what type of woman she is. Either she is an attention seeking wh0re, or she is loyal to you and ONLY you. There is no such thing as being in-between. This is where you need to stand up and decide for yourself what type of woman you are with, and if she is worth any more of your time.
Laterz...