Biphobia?

So I've been dating my first girlfriend for 3 weeks and it was her birthday yesterday so I met some of her friends and at one point in the night somebody asked me if i was gay or bisexual and I said bisexaul (because that's how I identify) and my girl friend said no your gay your dating a girl so your gay and I tried to calmly explain that I'm still bi no matter who I'm dating but she wouldn't listen and I didn't want to argue with her on her birthday in front of her friends.