What if the male is single and the female is not? Here is the classic case of the tease. In situations like this, the male is usually dragged along by the female, listening to her complaints and dealing with her bull shit. But, do not be fooled. The only reason a guy sticks around for that cruel and unusual punishment is in hope of a crumbled self-esteem or relationship for the girl that leads to a hook-up.
What if the female is single? This is a tricky situation because each guy is different. But, I would have to say about 70% of men would hook-up with a female friend even if they were in a relationship. I am in the other 30%, but I have seen first hand the majority side of this figure.
Lastly, what happens if both people are in relationships? Only one thing matters in this situation: Do you generally give a shit about your significant other? If no, the relationship status has no bearing and sexual temptation will take over. If yes, I say the friendship stays as just a friendship.
So where do I stand on the issue? After looking at each situation, I am going to have to agree with Chris. I think it very difficult for a male and female friend to remain friends without any other feelings getting involved. The only way this does not happen is if both are taken and happy to be taken, which, I would have to imagine is not the case until these "friends" are looking to tie the knot. So, for all you high school guys and girls, keep an eye on that close friend of the opposite sex that hangs with your significant other. Odds are, they have other intentions.