It is quite difficult for a girl and a boy to remain "just friend". That's not what I say. You can be whatever you want to as far as I am concerned. Friends with benefits. Friends without any freaking benefits. It is your freaking call.
However, there are a certain reasons why humans of opposite gender cannot be just that...friends. There are good reasons for it. The complications it carries as a baggage is another thing to mull over. For now, let's discuss the reasons.
Here is something to think over...
The friends with whom you can hang out with anytime. The ones whom you chat away for hours without keeping a track of time. It all changes when either of you get into a relationship. But, you continue to keep them close to your heart.
You guys are aware of each other's feelings but either the ego or the hesitance to get into a relationship is stopping you both from doing anything about it.
You guys get together just for some fun time or for hanging out with each other. There is no sort of complication going out for you guys.
...if you know what you are doing go with it. But, remember do not sabotage your happiness for anything. Anything.