kind of unfairly judging her for having sex with someone else when you two "broke up" and ceased contact

Just to be clear here - absolutely none of this is due to her having sex with someone else. I'm not even sure it happened. It's about the fact that I tried for a serious relationship, got denied on the grounds that she didn't want one, and then she went out and tried to get one anyways.

The only reason I even mentioned the sex thing is because it punched me in the gut, I wrote this mostly just to vent, and I needed to vent that detail in particular.

I actually responded to a comment similar to yours below, I actually agree with you on the premise that she might have only realized that she was ready for a relationship after we stopped talking and she went on some dates (dates that were potentially just a setup for a FWB).

My main issue though still, is that we didn't even do the "date" thing really. 90% of our interaction was sex. We only hung out in a "date" setting one time.

As I said I still haven't closed the door on her yet, and comments like this are exactly why, I didn't even realize this possibility until someone said it (I was a little blinded by my emotions).

I will discuss it with her, but I'll be honest, if she just said no to me for the serious thing, and then went on dates looking for something serious? That's probably where my whole "I refuse to be someone's second choice" thing comes in.

I actually was pretty blunt with her and already said most of what you said in the first paragraph. "I can't be friends with you because even if I could handle not having you, I know you have a high sex drive and need to have that taken care of, and I can't hang out with you and some guys who's fucking you, that would really hurt."