The issue with this question is that everybody has their own definition of friendship. For some a friend already is the drinking buddy who you see at social meetups, to others it has to be somebody you have to interact with every day. In reality, you really cant have more then 5 "real friends". If you think you have 20 friends or something you are going to be dissappointed when you age.
I personally think that, especially at a younger age, a close friendship between two people of the opposite gender is close to impossible. I honestly cant think of any "close friendships" between 2 opposite sex people in my real life were one actually isnt interested in the other.
If you would bang your best friend it is already not a normal friendship.
One thing that can happen for sure is that one had a crush and lost interested over time.