I think it is possible for men and women to be just friends...only up to a point. After you reach a certain point of closeness and friendship, almost for certain at least one of you is going to starting thinking or hoping that there could be something more.
I'm now paranoid about getting too close to a guy even just as friends, and what can happen if you try to take it beyond friendship. I started going out with a guy, four years younger than me (something I never thought I'd do), I was really good friends with--we'd known each other a bit over a year, but we'd been close friends about four months, I'd say. We went out for four months, and after the first month he started telling me he wasn't happy 24/7 like he expected to be so he had doubts about whether it was the right thing. So we'd have these crazy conversations where in the same breath he'd tell me it wasn't working but that he loved me so much. He showed how fickle and immature he was, and ever since our friendship has been on the rocks, though at least now we're talking a bit again.
Be careful about those friendships, I say...