Here is a website that deals this this question:
http://www.intellectualwhores.com/masterladder.html
Here is the foundation of what the Ladder Theory is based on:
Sally: We are just going to be friends, OK?
Harry: Great, friends. It's the best thing...You realize, of course, that we can never be friends.
Sally: Why not?
Harry: What I'm saying is - and this is not a come-on in any way, shape, or form - is that men and women can't be friends, because the sex part always gets in the way.
Sally: That's not true. I have a number of men friends and there is no sex involved.
Harry: No, you don't.
Sally: Yes, I do.
Harry: No, you don't.
Sally: Yes, I do.
Harry: You only think you do.
Sally: You're saying I'm having sex with these men without my knowledge?
Harry: No, what I'm saying is they all want to have sex with you.
Sally: They do not.
Harry: Do too.
Sally: They do not.
Harry: Do too.
Sally: How do you know?
Harry: Because no man can be friends with a woman that he finds attractive. He always wants to have sex with her.
Sally: So you're saying that a man can be friends with a woman he finds unattractive.
Harry: No, you pretty much want to nail them, too.
Sally: What if they don't want to have sex with you?
Harry: Doesn't matter, because the sex thing is already out there, so the friendship is ultimately doomed, and that is the end of the story.