No matter if she cheated or not. As far as my experiences go it is not normal at all to just set up a date with another person you are attracted to when you are in a relationship. At least not without talking with your partner beforehand.

What would you have done if she told you in advance, without lying, that she's attracted to that guy from the houseparty and wants to have some drinks and a sleepover with him? Also having fantasies with him?

Honestly if my partner came up to me and tell me that I would think he's kidding. Also it would make me anxious and jealous all the time around other women. Not a good base for a long lasting relationship. But that's just my personal view of course.

To sum that up: If you are not okay with your girlfriend dating other guys (because that's exactly what she did) and lying to you then you should end the relationship. But I can full on understand if you can't do that right away.

So - If you choose to stay with her for now, have a deeptalk with her and set clear boundaries with what you can and can not live with her doing while being in a relationship. That deeptalk should include her feelings for you and how hurt you are by how she was handling the situation. It may sound hard but sometimes people don't have the strength to admit that they don't have feelings for their partner anymore, and you are already in a one sided relationship for a while. Also Stay aware of more red flags and if you catch her lying one more time, cut her off immideatly.
But that doesn't mean I wouldn't suggest dumping her right away. Best of luck to you!