Is OTHER wrong in the post ? POST : My Ex and I have not been together for 4 months, but have stayed good friends. 

I made a friend thru him, who became a pretty good friend of mine as well. Her and I hungout all the time. and since She is also my ex's roommate, I would vent to her about him and she would understand exactly where I was coming from.

I recently found out from Facebook that my ex and my friend are dating. To say I was livid is an understatement. I felt betrayed and ripped both of them a new one. I'm not upset that they're dating. I'm upset they couldn't give me the respect to tell me in person, or even send a text! 

I felt like they snuck behind my back and started this relationship. both my ex and my friend told me it was none of my business that they decided to date. but I think if my ex and my friend decide to date, I should have some sort of knowledge about these feelings that have developed. and they've it's childish I no longer want to associate with them. Mind you both my ex and friend knew this has happened to me in the past (a person I thought was a friend sleeping with an ex) so it just adds to the hurt I feel. 

I'm not jealous and I wish them all the best, but AITA for no longer wanting to associate with them? are these valid reasons to cut ties with people?