Should I feel bad at all?

First off Sorry for the long Message:

My ex-best friend broke up with another mutual friend about 2 years ago. I was friends with both of them before, during,and after this very very messy break up. My friend (whom I will call Sherry for simplicities sake) went on to do her own thing, joined a sorority, got a little crazier (if you know what I mean) and became I person that I still loved but was completely different when she hung out with other people; she became part of the crowd. We were friends for 6 years. I am also friends with her ex-boyfriend, we were friends before I met her, we have know each other about 7 years now; after the break up he joined the marines and is now in Iraq. Before he left however, me and him became friends with mbenefits. This apparently is a problem.

   Before I left for a weekend to meet with the marine I met up with Sherry for the 1st time the whole summer, although we do not live very far from each other. While she was doing her sorority thing in NC we barely spoke, at one point we didnt speak for 2 months straight, very weird since we spoke about once a week. I began to loose touch with her especially since a lot of things happened that I wanted to tell her about but she was never available to speak. Regardless of this when we met up I told her about me and the marine--she said it was fine although she was surprised. She and my marine friend where not talking anymore as she broke his heart yet  again two weekend before this when he told her he still loved her but she said she didnt feel anything else for him That weekend I left to meet with the marine and we spent 3 days together. ahem. 
The monday I come back my marine friend calls me and tells me that Sherry had messaged him telling him that if he though that there was any possible chance of them getting back together it is completely gone because he slept with me, her best friend. That infuriated me, of course. That same day she asks if we can meet and I agree to, when we meet I am already angry at her but she proceeds to tell me that she thinks that I made a wrong decision and that I wronged HER! I was confused and angry; I told her that the people I sleep with is my concern and that this has nothing to do with her if she didnt have any more feelings for my friend. She said she didnt have any feelings for him at all but that my act alone was completely wrong. Furthermore, I asked her to stop being selfish and thinking that everything was about her (she told in our 1st meeting that she though another one of her ex-boyfriends still liked her). 
 I got up, paid for our drinks and let...but came back from the door because I didnt think this is how so many years of friendship should be left. When I sat back down at the table she was still there and had dialed another friend to gossip about me (I wasnt even out the door!) I told her that actually we were not done. She kept telling me that what made it worse was that I didnt even think there was anything wrong with my actions and that until I accepted what I had done she had no more words for me. I told her that I would never giver her an ultimatum to our friendship and that never once had I told her that until she accepted something I didnt agree with (even through her various bfs and cheating streak) couldnt proceed to be friends with her. She finalized with saying that she didnt know what planet I lived on where doing this was acceptable and that she had no more words for me. I said, "in one that does not live in the past, in one that making myself happy is a priority." 

I knew that we had been becoming distance for a while but she never thought that. Should I feel bad? I dont. Was it really wrong to sleep with my friend?