Interesting review of the Ladder Theory. this video is, of course, far from being a reliable source of information. It's highly editing and assumes to much, but it illustrates a point.
The ladder theory states that woman have two ladders, and men have one. These ladders are cataloging systems for intersex relationships. The woman have one ladder for potential mates and another for support and friendship. Men, however, only have the potential mate ladder. Thus, the higher the girl climbs on the guys ladder, the more he wants to date her. However, the woman can befriend a guy without the same problem.
While this theory is far from being proven, it does have it's representation in biology. The male must spread his seed and always find potential mates to propogate good genes. However, the female must care for the young and is thus generally monogamous, with a small range of potential mates. Instead, she must find support systems and people to help her care for the young.
So while a guy can be supportive and helpful, non-aggressive toward his female friend, the idea still lingers. I can't personally deny it, there are a few close friend I have that I would readily hook up for if not for my current relationship. (The current relationship is far more valuable, of course). It's an interesting theory, but not yet solid fact. If you wish to deny it, you still can.