The true answer is...
YES. Absolutely. It is very easy for members of the opposite sex to be just friends.
The reason we feel this is a tough question to answer is because, for alot of individuals, opposite sex friends are immediately considered possible mates and the individuals with this perception allow this outlook to get in the way of the possibility of true friendship.
When we are young, the opposites of our sexes are based on our upbringing and mostly don't become a factor in deciding friendships. Granted, boys like boy stuff and girls like girl stuff, and in most cases, either sex is not attracted to the other simply because of sexual expectations imposed on kids. With this imposition removed, the differences are rather mild and friendships are easier.
As kids move up in age to pre-teen, the impositions are easier to ignore and this age area opens up for quite a few opposite sex friendships.
Now, the teen years is where situations can start to become questionable. Changes in the body and attractions to the opposite sex go haywire. Many who could be friends before begin to question attractive elements and can no longer "just be friends."
This is not always the case, though, it depends on the individual and their comfortability in their sexuality as well as being able to discern themselves responsibly enough to maintain a friendship as being just a friendship.
Becoming adults, the road to maturity is broadened - on one side, a person can decide to be mature, where as on the other, maturity takes a back seat and a person decides that, now that they've reached adulthood, they can do whatever they want. Unfortunately, most don't mature for a while, if ever.
Once the obvious is confronted, a friendship can be just that. I've seen proof of interactions between friends that don't involve their sexuality past the point of he being him and she being her.
In the long run, people just need to grow up.