My (22M) girlfriend (18F) of 2 months ditched me at lunch to eat with one of her "guy friends" that also happens to be her ex and says I'm overreacting.

It was at a church dinner, she basically stiffed me to sit with her ex, after we'd spent the service sitting together. Anyway, when I confronted her about it she was unapologetic and acted like nothing was wrong and I don't think I can invest myself in someone that acts like emotional infidelity is no big deal.

tl;dr Girlfriend made me sit by myself at a church dinner so she could sit by her ex.

Is there any reason not to break off the relationship right now?