Throwaway. I'll keep it simple: My girlfriend threatened to withhold sex until I apologized for an earlier argument.

Me: Don't threaten me. I will have sex tonight whether its with you or not.

Her: *pause* Fine. If you cheat then I will assume we are no longer monogamous and act accordingly.

Me: Are you threatening to cheat on me?

Her: Use a condom. *She then walked out*

I am aware I did not handle this very well. I need to know how not to fail this relationship test. Obviously I don't want her to cheat and I don't want to end the relationship over a petty argument. But if I back down and apologize I'm acting like a beta and leading her to cheat anyway. Where did I go wrong here and how do I fix it?

The argument was her being emotional and I escalated. Unfortunately I messed up somewhere. Possibly escalated too far? I left for work immediately after she walked out and we haven't spoken since.