I've always had close female friends. When I was younger I had more, but overtime that naturally shifted.
I know the potential pitfalls of being in a very close friendship with a woman. I know because I've only had one female best friend in my entire life, and I now have sex regularly with her, and she wears a silver and blue diamond wedding band on her finger. So I've been there, done that.
So while I've never had another female "best friend", I have, and do, have real friendships with women. Some of them I'd deem unattractive (by my personal likes), and others are drop dead gorgeous (I'm an actor, it's an occupational hazard

).
My wife and I never put boundaries on friendships (well save one), and we do not now. Our motto is, and always has been, full trust. We both believe that if one of us ever cheats, as improbable as that feels to both of us, it'll be because we chose to cheat. It won't be because of an opposite sex friend, or anyone else's fault. It's us. Period. We are responsible for our marriage, and nobody else. The way we define our relationship (and again I am not speaking for anyone else) has to allow for total freedom and trust. Anything less, and we'd be living in the kind of relationship that neither one of us wants to be in.
Having said that, what is interesting is that there are natural boundaries that exist. We both are fully allowed to spend one-on-one time with OS friends, and we do occasionally, but not nearly as frequently as we do with our SS friends. Neither one of us has taken a trip with an OS friend, like we have with SS ones. I don't have a single female best friend, but while my wife does have one male best friend, almost all the other "best friends" are of the same sex. It seems that divisions, and boundaries, happened naturally, without either one of us needing to put our foot down (again, except that one time).