The mythical unicorn of the guy/girl platonic relationship is actually possible -- you just need an ironclad assurance ugly-bumping'll get ugly. Here’re the few factors potent enough to keep you mercifully unlaid:
1. Track Record. Remember that guy she dated who got her banned from Facebook for harassment? Or the one whose grandmother's funeral she crashed when he wouldn't text her back? How about the dude who became a monk? Do you want to be the new those-dudes?
2. Family Ties. You don't want to explain to your best friend why his sister's car is randomly parked out front, or freak your nephew out with a "when your uncle and Ms. Peterson love each other very much, or don't, but happen to be bored" talk.
3. Tough Act(s) to Follow. Didn't she used to bang Channing Tatum? Yeah, the abs dude. Yeah, G.I. Joe was a good movie. How's your six-pack these days, bro?
4. Climax Anti-Climax. When it’s like watching the movie after you’ve already read the book -- you just know each other too well.
5. Friends with Experience. Heed the comments from friends who've already blazed this trail -- "rude service" ain't just something to avoid on Yelp.
6. Prior Skankiness. It may not be fair, but you just can't wish her past away. Or herpes.
If all else fails, your relationship is still safe for the simple reason that, if you're friends, she's already decided not to sleep with you. Seriously: what guy hasn't overlooked a minor case of psycho-familial-celebrity-banging-overfamiliar-sexually-selfish sluttishness? And you wonder why she won't sleep with you.