It takes a certain kind of guy to make a good platonic friend.
What the guy must know is, the reasn why you are so close as friends is because you are not a couple. That does not translate into - because we are so close, why don't we become a couple? Sadly, a lot of guys don't realise that simple fact, they try to 'move' the friendship on you a different level.
How do you avoid this? Watch for the signs. Is he appearing jelous about someone you are seeing / talking about? Is he getting possessive about your time with other people? Is he moaning about being single? All these may mean he is about to try 'the move'.
Your response? Underline the value of your friendship, because it is a frienship, and it wouldn't work any other way. If he is receptive, he will understamd what is being said, and back off, if not, then you may loose your friend because he will step over the line, and you may not be able to step back.
From a man's perspecitive, almost all my closest friends are women, and I'd go with any of them in a heartbeat BUT I am very very careful to never ever let them see that, and to know that we are friends, and going to stay that way, and I'm not going to spoil it.