It seems like a lot of the people saying no have some pretty odd ideas about what the friend zone actually is. It doesn't have anything to do with feeling like you're "owed" sex for being nice. That's ridiculous. It's as simple as wanting to be involved with someone romantically, and the other person just wants to be/stay friends.
Having said that though, while someone can put you in the friend zone, staying there is a choice you make. Unfortunately when I was younger, I didn't really get that. Whenever I was turned down by a girl I liked, I would accept being in the friend zone and become an orbiter to avoid damaging the relationship any further so I could maybe try again later. Of course it never works out that way. Once someone categorizes you as a friend in their mind, that's most likely all you'll ever be. Worse still though is the knowledge that it's not that they really want to be your friend, but rather that they don't want to be more than friends. Hmm...it really is a pretty backhanded thing now that I think about it. Anyway, eventually I got the old "let's just be friends" routine one too many times and said to myself, "You know what? **** her. I've got plenty of friends, I don't need another one." And I moved on and forgot about her, and it was great! That's kind of been my policy in these situations ever since, and I have to say it's worked out pretty well.
Residents of the friend zone, learn from my mistakes; You can move out at any time.
There's a lot of girls/guys out there, and guess what? Not a single god damn one of them is worth the heartache. Just let go and move on. Don't be an orbiter.