Well, you know how I feel about this. I've had a female best friend for donkeys years (and I have a few other female friends too). But nor do I feel the need to argue with you at any great length, as I daresay you have a point in general terms. All I can say is, it works for us (ie me, my wife, my friend, and her husband, with whom I am also good friends - or perhaps I should simply say we are all good friends with one another). My wife actually credits my best friend with improving our marriage because she often asks me to see things from my wife's perspective, and my best friend's husband has said similar things about me. But you may well be right about the exception that proves the rule.

On a sort-of related note, regarding what you said about people having designs on one without one knowing, because I'm quite a paranoid person, and because I typically have no idea if someone has designs on me (a rare occurence, unless the person is insane, I daresay), I always get the opinion of my wife and sometimes also my best friend, both of whom are far more attuned to anything untoward. In this manner, on two separate occasions over the past twenty years, they've warned me off a couple of people based on what I told them they said, or based on emails etc I've shown to them. I'm very glad I have them both on side for such blind spots, though they have been rare (as I said, a mere two in the last twenty years, and both were nipped in the bud once they told me my paranoia was justified).