I refuse to believe in the existence of a purely platonic friendship between a guy and a girl. It just cannot exist. At least not from the guy's side. Unless he's totally and unequivocally gay!
If you're a girl reading this and going - "Aw, c'mon! You've got to be kidding me here! I've known Paul since we were both in diapers and I'm positive he doesn't think of me that way at all!"
Well, there are only two things I can say. Either you're bald with a hunchback and a broken nose; or HE'S GAY!!
The key point is that a friendship between a guy and a girl is viewed differently by both of them. The best way to put it would be as follows [I remember coming across this on someone's blog, but I don't remember whose] - the girl sees the guy as a 'friend' first and then as a 'guy', whereas the guy sees the girl as a 'girl' first and then as a 'friend'.
So, to a girl, a close male friend differs little from an equally close female friend. But to a guy these two things are quite different. This explains why guys are more willing to open up emotionally to a female friend than they are to a guy buddy.
Now, to get back to my main point - I'm NOT saying that every guy wants to sleep with every girl he knows. I'm just saying that he doesn't view her in a totally platonic way. There must necessarily exist a modicum of 'unplatonic-ness' from his side. Exactly how much of it is present is largely dependent on the situation and the people involved. But it is there.
Another point that I'd like to state is that I am not speaking solely for myself. What I am saying holds for all straight guys out there. Any guy who denies it, is just doing so because he is afraid to admit that it is true.
Whether or not the same feeling of unplatonicness is also present from the girl's side, is not something that I would be most qualified to comment on. I would expect it to be true of the girl too, but I wouldn't be surprised if it isn't. Girls are just different that way.
It's important to realize that there is nothing wrong in this. It's just the way guys are, and girls will just have to live with it.