Let's paint a picture where you have a girlfriend - congrats by the way - who you've been going steady with for a couple months. Her parents adore you, you're comfortable with one another, she buys you FIFA 13 and pretends she knows what she's talking about - it's adorable, it's simply paradise.
Now let's paint another picture right next to the first - in this very special picture, she's invited a friend to that new Twilight film she dragged you to see. So you show up and it turns out the friend is a guy, which is fine.... right? It is, as least until they're making inside jokes and he finds his way into all her pictures. Now, you don't want to get that 'worst bf in the world' status so you keep quiet. At this moment in time, all you can really do is tweet your feelings and reblog those heartbroken tumblr pictures, maybe take a stab at some poetry - like a real man.
Wait a minute! That's YOUR girlfriend. You should have a say in the friends she chooses! You march right over there and take her back... congrats, you're single again. You see, this concept of men and women genuinely having a strong, strictly platonic bond between them has been argued for so long that it brings to question whether we as a people really have interesting things to talk about any more. This newcomer is seen as a potential threat due to the fact that society has warped us to believe that a man and a woman cannot be best friends, therefore, you get jealous. However, this all boils down to whether or not you truly trust your partner.
Sure it's natural to feel jealous: but when you're willing to tear apart a newfound friendship because you don't feel safe, it's time to ask yourself: do I trust her? If not, you're better off single, pal. First, how about you give the guy a chance? For all you know you might like him too. Also it permits you to see the grounds of their relationship, as in: why your girlfriend likes him so much.
Granted if it is more than it is, then confront her, otherwise: what is the problem? Let's be realistic: you may be an important part of her life, but you're not her whole life. Just give her some room. If you do feel left out, telling her about it might make you feel like less of a man but at the same time you might just end up keeping your girlfriend. It only makes sense that communication should be the basis of any relationship. Instead of walking into a shouting match that you would probably lose, because...well... you're a man, why don't you sit her down and talk? If the relationship is as good as you tell your friends it is, she will understand.