The term a Guy or Girl uses when rejected by someone they fancie/like/love etc... Generally used to obtain sympathy.
The notion in its self was probably conceived by someone wallowing in self pity after being "nice" didn't get them sex (which undermines the point of been nice) they also see people succesfull at getting relationships with women as "Jerks" etc... when in fact they are just jealous.
Being nice to a girl doesn't mean they owe you sex, and if that's all you want from them then your not the "nice guy" your in fact a "Jerk"
The girl/boy isn't "using you" when in the friend zone. you are free to leave whenever you want but they probably do stuff for you to that you don't notice seen as your so obsessed with having sex with them like help cheer you up when your down, they might do you a favor every now and then like any other friend of the same sex.
If some other person gets the one you like don't think you are the only person that has true feelings for them. your feelings are no different than anyone elses
There is probably people you have freind zoned so dont act so innocent either