"cheating" can mean a lot of different things to different people and can be a large number of situations, so it can also mean a whole lot of completely different things.

The cheating isn't the issue, though. That's a symptom of something. The main issue is whether you want to be in a relationship with her and if she wants to be in one with you. That's all you need to figure out.

I honestly don't understand monogamy, though. How does her having sex with someone else substantially change anything? It can be a symptom of something, though. For example if she had a long drawn out affair with someone else with emotions involved, that's a pretty good indicator that she is not getting something from your relationship that she needs. However, if she just got drunk one night and ended up with a guy in a hotel room, it may not mean anything and isn't really a big deal as far as I would be concerned.