Desert imbues value in a thing or person in a way that ultility does not. Utility measures the value of a means while desert measures the value of an end. Utility allows for the thing to be consumed in the service of some further, ultimate end; desert demands the thing or person be honored and consecrated.
Prostitution is the shift in the valuation of a person or a calling from desert to utility. Utility can be alienated and traded; desert is inalienable and subject to tradition, to traditing.
Dignity is lodged in the sort of value that is measured by desert rather than by utility. Prostitution is an affront to dignity.
Dignity stops the flow of evaluation from moving toward some ultimate, higher end and forces it back to the appreciation of the sanctity of the particular person or consecrated object. This stoppage of the flow of evaluation resembles the stoppage of the flow of responsibility that is the essence of the meaning of duty. (Duty is the taking of responsibility rather than trying to palm it off elsewhere.)
Desert allows for intermediate values to become ultimate values.