Im bisexual with a girlfriend, my friends are acting awkward around us is that normal?

So I am only 15, I know I am bi and I told my friends when I started dating my girlfriend whom I like very much they started acting weird when ever i mention her and way weirder when shes around it has gotten to the point that my girlfriend doesn't want to hangout with my friends because she doesn't feel comfortable and like she fits in... why might my friends act like this? and why is it any different then if i were to date a guy? How can I make them see its not a big deal and to just accept it?