Sex Weakness

About 5 months ago a friend of mine brought along a girl she knew from her neighborhood to come party with us. We talked a little, drank a lot, and she ended up in my bed with me that night. The next day she left and i didnt know her name and really wasn't all that concerned if id ever see her again. Later that day she sent me an e-mail and we started talking a lot and i learned that she was actually a really cool person and i became a lot more attracted to her.
We learned that we had a lot in common and i started to think about her as more than just a one-night stand. I actually began to like her and care about her. she told me from the start that she'd just gotten out of a 5 month relationship but the more we got to talking, the more i noticed that her life with her ex was still very complicated and i realized it was not a good situation. Regardless, we continued dating for a month more or so until she finally freaked out and said we could only be friends because she didn't want to lie to her ex about seeing someone new -- it seemed that every time the sex got better, she got more and more scared.
We haven't had been intimate with eachother for almost 2 months and are trying to just be friends but last night after talking about how sex would be bad the next time we see eachother, the conversation quickly turned into some really good phone sex. What have i gotten myself into? Is this a fling? Are we better just as friends even though the sex is so good or does it seem that we might have something real together? My friendship with her is really important at this point and i dont want to lose her but i know that we're both weak for eachother. What should i do?

- Miggzy