To fascinating to fuck? No way, doesn't happen. Maybe there is someone out there who has no interest in sleeping with you, speaks so well and has such an interesting life that just hearing their voice drops your draw and keeps you entranced until they are out the door and your sexual desire returns, but I'VE never seen one.
What you are talking about is fear. Forget that "It'll ruin the friendship" bullshit. I had that attitude about this one girl. SOOOO much sexual enegry between us, but I didn't want to ruin the friendship. Guess what? We haven't spoken in years. You never know what will end a friendship, you might as well be honest with each other....that is what friendship is about anyway.
Sex is one of the greatest forms of appreciation out there, show them how fascinating they are by trying to give them the best sexual experience they've ever had. That will NOT ruin a relationship. Isn't the person you spend your life with supposed to be your best friend anyway?
As long as you are honest about your intentions, don't play games and discuss what you want then any bad feeling resulting from a break-up will eventually disolve and you will be able to be friends again.
Unfulfilled sexual desire has a way of driving people apart too. If you can't control yourself and don't want to "ruin the friendship" you stop doing things with them for fear that you won't be able to control yourself.
Go for it! If you get turned down, big deal, get over it and stay friends. If you hook up, just be honest and things will work out.