Back when I was single and had male friends I enjoyed the casual companionship they often provided, but I noticed that inevitably that easy friendship would get more complex over time as one or the other of us would stop seeing the other as a friend and start seeing a love interest. Then things became awkward, unpleasant, and sometimes rather ugly. This happened time and time again, and it happened even when he was involved with a friend of mine or when I was seeing someone else, or when I thought I had been unambiguous about where this was going (as in: we were not going to be having sex together at any point in future) and had been conservative in dress and comportment. It just tended to get messy over time.
Twice I had roommate's boyfriends scope me out as to whether we might have a (hypothetical) chance together in future, should things go sour in their current relationships. (As I am not a man poacher, I had to come up with a way to dodge without being insulting or triggering a tattling reaction - tricky.) Once I had what I thought was a very positive friendship with a coworker. This was while I was living in Russia. The lifestyle there was very hard, and while we had very little in common personally and were both in long-distance relationships with other people, we enjoyed each other's company and provided each other a good ear for venting about the frustrations of living and working as we did. Then I took a trip with him over the Christmas holidays. It was open invitation getaway, and a third person considered coming along, but didn't in the end. This was supposed to be a weekend thing - see Vilnius and get back home - but I had visa problems at the border and wound up stuck there because the consulate was closed for winter break. During all of this, things came to a head in our friendship. He made a pass which I deflected, and then he bailed out of the city and left me to deal with all of these problems alone. Somewhere in the middle of this frigid, frightening sojourn which included getting shaken down by tourist-targeting thieves, being asked to pay for my new passport photos with sex, and playing hardball at the Russian consulate, I began to realize that this relationship had been doomed from the beginning because sooner or later one of us was going to cross the Rubicon of loneliness/horniness and an act of war would be committed. Which is exactly what happened.
After that I revised my expectations of friendship with men down to a much more manageable, less vulnerable level and at the first hint of awkwardness tried to limit contact so no one would get hurt. Eventually I came to the same conclusion Harry does above and resigned myself to more shallow social interactions with men. Now of course I am married, and I forego male friendships because it might give the appearance of wrongdoing or expose my marriage to stress it doesn't need.
I'm not saying friendship between men and women absolutely can't be done. Perhaps other women are more socially adept than I was and can better sidestep the hidden land mines of sexual attraction, rejection, and hurt feelings than I could. I will say that my best friend in high school was a guy. I spent loads of time with him, and things never became awkward between us and I still feel wholly positive about that relationship. He was, however, gay.