Even though you may say you are just friends, when you are with a women intimately, she is probably in the hopes that you will fall in love with her. If you are going to have a one night stand then, do it with a stranger, not with a girl who is already your friend. Because, if she is your friend then, she already has some kind of feelings for you and to sleep with her will only make her feelings stronger.
"Do you believe in love at first sight? Or should I walk by again?"
"Some people believe that holding on and hanging in there are signs of strength. However, there are times in life which it takes much more strength to just let go"