It depends on the mind-state of the participants. If one happens to be homosexual, it would be easy to be friends forever. If both are straight, but have little interest in sexual intercourse or any other sexual interactions, then it would be easy to remain friends for a very long time. It gets harder as they age. Getting older, one may see the person they looked at as their best friend as a potential date, which may cause problems. Other occasions may lead to those looking at each other less sexually as they become more older and mature. Inter-gender relationships can never be guessed upon.