It happens but either he's gay/taken or a complete puss,thinks it cant happen and/or she's unattractive,too attractive, gay or an ice queen. There are exceptions but thanks to the fact we are all just animals and our animal brains are wired for reproduction as a priority no man and woman can truly "Just be friends" unless they fall in to the previously mentioned categories. Especially if they spend tons of time together.It will come out some day. At a party, wedding, reunion, or something. A little booze and some privacy and a perceived opportunity like a breakup and it will come out. Its inevitable. Some exceptions,not a lot. Sex is the main reason for everything we do. You spend all sorts of time on hair,clothes,your physique,cars,houses,boats,and the like to get approval from the opposite (I guess) sex and get you some. It why people do everything.