Hope you used protection OP

Hey...but you tapped that...

Not your fault, she's the one sneaking around
Probably best to cut ties with her, the person she's dating will see her true colours soon anyways

So you hooked up with a girl you had a crush on and found out that she isn't actually faithful towards the people she dates?

I'm not sure where the FML is in this story. You got laid AND dodged a bullet.

It’s probably that he learned the girl he had feelings for was not what he thought she was. And it probably has something to do with the fact that he was an accessory to cheating

At least you found out before it became emotionally really tangled.

Are you sure that sleeping with other people is against the rules of her relationship? Yes, it's often the case, but it doesn't pay to make assumptions. "Cheating" means breaking the rules, and she might not have actually done that.

I once explained to my grandfather that polyamory involves seeing more than one person at a time. He replied, "Oh. We used to just call that 'dating'."

You learned to not commit to trash and now you can move on to someone worth the time. That's a good thing in my book.