Friends with benefits with an ex. Asking for trouble?

My relationship (of about 4 months) as succinctly as possible: we became very good friends and the sex was awesome. She expected a lot, I couldn’t deliver, and she blamed me for not trying. The constant disagreements and her emotional instability made me realize that it wouldn’t work in the long term, so I broke it off.

I miss the friendship we had but she claims friendship is too painful at this point. However, she wants to continue having sex, with the only rule being that we have to let the other one know if we start dating someone else.

The problem is, I know that she’s lonely and wants to have someone to talk to on the phone and hang out with. Add sex to the mix and it looks to me like a relationship with a different name. I told her that if she wants the sex, she can’t have the other stuff. After all, intimacy both inside and outside the bedroom will create or sustain romantic feelings, wont it?

She was upset that I suggested we limit our other interactions if we’re going to do the casual sex thing. I’m trying to protect her feelings as well as my own. Is this whole thing too messy a proposition to begin with?