Northwoods is correct. Humping is not usually sexual behavior, but dominant behavior displayed by both males and females. She is doing this to you not out of love, but to control you. She doesn't see you as the Pack Leader - she's taken on that role for herself. Not good.
You need to learn how to be the Pack Leader if this behavior is going to stop. I strongly suggest you buy, or check out from the library, "Cesar's Way" by Cesar Milan - the Dog Whisperer. I know from personal experience his techniques really work. Give it a try!
When a female (or sometimes a male) dog humps like that it is a domination behavior.