Is it normal i fantasize about my banging my girlfriend's friends?

You could ask her not to tell you stories of their sex lives.

You should think about you're girlfriend more than her friends, it'll be best for you.

It's very normal and it's fine as long as you don't do it for real.

Tell her NOT to tell you those stories because then it's harder for you not to think about it.

Your girl is YOUR GIRL. Only because you're a guy that doesnt mean you have the right to engage on those thoughts. Take pictures of your girl, ask her for naked selfies for example? That way you only think of her.

Why on Earth would she put images of her friends getting fucked in your head..? I think she's the one who needs some advice. What a careless thing to do.

Like noid said, you should (seriously) ask her to stop telling stories about them, unless she wants you to think of her friends getting fucked. Quite frankly, it'd be kind of wrong of her to completely hold it against you for having fantasies about her friends. She is the one who put them in your head.
That being said, you should try to show restraint when you do start thinking about it. Try to think of something or someone else to fantasize about.

Probably normal. She might fantasize about riding your bro's dick.

Well now you have something to tell her if she asks about your fantasies.

Ask her for more stories. They are nice fantasies that the two of you can act out with each other.

Stick your cock straight up her candy ass all the way to the hilt and then ram your dirty cock and nuts down her throat and make her suck it until its clean as a whistle.

fantasizing is fine just be careful to put yourself into situations where it might step into reality. ie "i was drunk" is no excuse for having sex with your girls friend.

I think you should tell your girlfriend to stop telling you these stories as she clearly has no idea that this is a topic not to be discussed with a straight male (only between girlfriends ;) Just tell her the truth that you are very visual (like most men) and her telling these things creates images in your head that you find hard to control. she clearly creates temptations for you and should stop this. I had this with my ex and he made a move on one of my ex girlfriends. She did not like him but our relationship was really damaged and never recovered. I was stupid and had no idea he had these thoughts. On the other hand he told me about his friend and how good he was to his wife: kind, caring, generous, coming home at lunch to have sex with her etc. I still dream about that man to these days.... Focus on each other cos all else is just an liiusion. Good luck!

She might also be thinking about someone in her work while having sex with you. No worry

I know you're right. But I can't stop jerking off thinking about them?????