Hmmm. I would like to interject an opposite view.
I don't really believe men and women can have very close friendships that are completely platonic. Unless either or both are gay.
Of course, you can have acquaintances/colleagues of the opposite sex that may develop into friendships, but I don't believe they can be the type of close friends that you can pour your heart out to without some element of attraction. All friendships are built on attraction in some form or another. Between members of the opposite sex, that attraction will, in part, have a sexual/flirtatious (call it what you will) basis. Could be mild and then disappear, could be mild then gets stronger and then disappears. B=Could be strong, in which case, the friendship would probably develop into a non-platonic relationship.
That does not mean to say that either party would have had to act on the sexual attraction; but I still think that at some point in the friendships there probably was some chemistry (which of course, may remain buried forever).
This probably stems from my view that the word "friend" is used far too casually these days, especially with FB etc. Real friends you can count on the fingers of one hand, two if you are lucky. The rest are acquaintances, colleagues, people you socialise with.