A spayed female dog that humps does it to show dominance over another dog or as a natural part of play. Female dogs, whether spayed or not, do not hump as a sexual behavior.
Humping is a sexual behavior for male dogs, especially for ones under one year of age that have not been taught that it is inappropriate. It is also a way that dogs assert the hierarchy in a pack. The dominant dog mounts the submissive dog to show that she is the boss. During normal play, dogs take turns mounting each other because it is fun. This type of play is similar to the excitement a dog gets from jumping and barking while playing.