This said by the best description in a movie and whether you like it or not for the most part it is true....
Sally: We are just going to be friends, ok?
Harry: Great! Friends! It's the best thing.
Harry: You realise 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's 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 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: Nuh, you pretty much wanna nail'em 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.