I do believe it's possible to have sex with a friend. As long as it's accepted by both parts that, even if we appreciate each other enough to give each other pleasure, the "Love" with a big "L" part isn't in the picture and there's no hidden agenda.
I don't see a problem whatsoever in caring about somebody enough to want to give them pleasure. At that point, having sex is simply a more intense sensation than going to the theater with that person or, even simpler, having ice cream together. It's something that, if both feel like doing it, will allow a new, higher stage of connection.
It's the perfect solution for somebody who want some "human warmth" but can't or won't, for any reason, get into a steady relation. That person don't want a simple "fuck-friend" with no human connection, that person want to feel appreciated and wanted. It's one more way to tell that friend you like her for who she is, no matter what. If she feels the same, why not? As long as everything is clear on what the expectations are, go for it!