Betrayal comes haunting

It probably shouldn't bother me but it does. For about 10 years of my life I was extremely close with my best friend. We lived a block and a half from each other. We considered each other sisters. I called her mom and dad, mom and dad. They took care of me throughout my teenage years, way better than my own parents did. We went through everything together.
Now we all know that when you are close friends you don't sleep with each others ex's, or even date for that matter. Especially when it was one of the others "big loves".
Now plenty of her ex's had approached me over the years and I never even thought of doing that to her. She was my sister. I could never betray her that way. Even know when we haven't spoken in well over 6 to 7 years, I could still never do that to her.
Tonight I found out that she had no problem doing that to me. I found out that she decided to sleep with my ex when him and I were broken up but yet her and I were still friends. She never told me that. In fact she never, and I mean never had a nice thing to say about this guy.
I feel very betrayed. I know that I wasn't with him and that by all rights both of them had any right to do anything they wanted to do. But what about that rule, that you don't sleep with your friends ex's?
Maybe I shouldn't be, but I am hurt.

#9 The Transition

I have never had such an odd transition of going from single to a relationship. I don't know if it is because I was single for so long, or what exactly happened this time. I had more people in my life that I considered friends, that actually got mad at me when I told them that I was in a relationship. Their reasoning.....they thought we had "something" going. One of them I was incredibly clear with, that we absolutely could NOT have a relationship. In fact I had said it over and over again. Another one had never even asked me on a date, so how the hell was I supposed to know? Another one, hadn't talked to me in a month and a half. Yes, we had went on a few dates, but when you don't hear from someone anymore, you figure its over. Right? His excuse, he got shipped to Texas for Army stuff. Well I guess I am supposed to just know that huh? Never felt a spark with him anyways. It's just been odd.
I thought that when your friends were extremely happy and falling in love you were supposed to be happy for them. When I date a guy/girl and we end up just being friends after wards, I am really happy when he/she meets someone. I want him/her to be happy.

I don't know I just think that good friends are hard to come by, and I am not willing to throw people away that easily, so I don't really understand why other people are.