Many people do use sex as bargaining tool, knowingly and unknowingly. And women seems to be the greatest offender, as men seems to be unable to live without sex.
Examples of sex as bargaining tool are such as sex in exchange of some present, sex in exchange for the man to do something, sex in exchange for the man to not to do something and many more.
It also include "do it and you get great sex. don't do it, you get a dead log'.
Personally i despise these sex manipulators. Are you prostitute or something? Sex should be something natural and enjoyable for the partners, not as a currency to exchange goods or services.
If the other party still have to sort of "pay" for the sex, why not he/she just go for professional services, which in all case gives better sex?
And a relationship where sex is used as bargaining tool will not last. It causes frustration to the one being used. Why get upset over something so basic? Just get a new one would be easier.
If the manipulator would like to lower him/herself to the level of a prostitute, then you will be one - totally dispensable when you are out of favor.
There is always someone better.
Get rid of your manipulating man/woman.