I started this thread because of of a question from a friend. Your answers, especially Mathew, were exactly in line with my thoughts.
It is next to impossible to have a friend who's an attractive lady around your age. You should never be alone with them.
Fortunately, the only one I broke that rule for was my wife. Something felt right and I knew I would never do anything to hurt her. We were engaged that same day and married shortly thereafter.
She is my best friend and only female friend.
Pax