Single men and women cannot be friends. At some point one of them is going to start being attracted to the other (usually, tho not always, the guy for the girl). This is not necessarily a bad thing, but it can mean the end of the friendship if the romantic feelings are one-sided. On the positive side many great marriages have come about when friends fall for each other.
Go read the Ladder Theory for an explanation on how sexes view friendship with the opposite gender.