These things can be quite tricky. To be honest, I've never quite understood the expression 'Friends with Benefits'. It seems to me that the best committed relationships end up being with people who you would consider friends. There should also be benefits. Lots and lots of hot, dirty benefits.
But, I do understand that in most of these situations we are talking about a non-monogamous relationship. Which, I'm assuming is your situation. The answer really depends on your end goal. If you'd simply like to stay on friendly terms after the benefits go away, here is the secret: don't be a jerk. Be kind, be honest, be transparent about your reasons and then do your best to not picture each other naked when you see each other in public. And certainly, certainly do not be jealous if and when they find someone else to, er, benefit.
Translating a fuck buddy into a significant other is tricky business, indeed. But actually, my advice here is not much different. Be kind, be honest, be transparent. Also, be generous with the orgasms. Make it impossible for them to imagine being with someone else. But most importantly, talk about it. Talk about everything. The worst thing that happens to friends who have benefits is that one of the friends develops feelings that they don't share. Those feelings evolve and grow into, sometimes, genuine love. To blindside someone with those kinds of feelings can overwhelm them.
So JUST TALK ABOUT IT. Okay? The kind of person who receives orgasm after orgasm from you, cares enough about you to be your friend, but then doesn't want to date you? Well, frankly, they're using you. If that's alright with you, stick around, by all means! Being used can be lovely sometimes. But once you have real feelings, you have to make real choices. For your own sanity and the sake of your friendship.