What a girl says when you have been friends with her for a while, but does not want to take the next step and become your girlfriend. Basically those works are a huge kick in the balls.
Girl: "We could start going out, but I don't want to ruin our friendship."
Polite refusal to engage in sexual activity with a member of the opposite (hell, this is the new millenium, or the SAME) sex.
Amanda turned down Melissa's proposition for a dirty weekend, saying "I don't want to ruin our friendship." Then she deleted Melissa from her Outlook contacts, her cell phone SIM chip and changed her phone number.