I think it is very possible (for some people) to have a friend or best friend of the opposite sex and not be attacted to him/her in *that way*.
My best friend since probably the 8th grade is a (straight) guy. We've never been in that awkward situation, it has just always been platonic. And I have a serious long-term boyfriend (not the same guy!) so you see, it can be done sometimes
I think it just depends on what the persons are like.
In answer to Bamboozled's question about making spouse end a friendship if it is of the opposite sex, I wouldn't even think of it. I'm not married, but if I did get married and my guy has a best friend who is female I wouldn't ask him to end the friendship. And I would hope he wouldn't as me to end my friendship with my male friend. That's like asking "pick them or me", and that is not cool!