Most Helpful Girl
Seeing someone isn't the same as being with someone. And it's fairly common for people to be seeing multiple people until things get serious and you have the exclusive talk. If you're dating multiple people you should be honest about it and ideally not have a physical relationship with anyone. Although she shouldn't have gotten angry at you for not being comfortable dating her whilst she's dating someone else. I think you did the right thing.1
Most Helpful Guy
No, that's not true at all. Dating here is no different to anywhere else. Most people at least try to be faithful if they bother getting into a relationship. Some people won't be faithful, some will be players, some will be inconsistent, some will be attention seekers, etc. It's the same the world over. Human beings are no more or less complex anywhere in the world, that's my experience.
It sounds to me like you did everything right and she gave you all the positive indicators that would make you confident in her being a dating prospect. She then blew it out of the water with her revelation about seeing someone. You then acted with dignity and respect and she turned into a crazy bitch. I would say that you have dodged a bullet.4