Good essays support the point of view of the author and submit to the fact that alternative views exist (counter-argument). I would suggest including a counter-argument in the essay to show openness to valid points which contradict the stand (keeping prostitution illegal).
For the counter-argument, one could argue that making prostitution would allow those who make a career out of it to file and pay taxes. The government could also place people in charge of prostitution companies which would insure testing was completed.
On the other hand, keeping prostitution illegal is best given:
-It is morally wrong to buy and sell sex.
-Adultery will skyrocket.
-It will confuse children about love and sex.
-The sexual pressure on women will increase(desire for women to model the behavior of prostitutes).
-If legalized, it will become commercialized. Commercials promoting prostitution will become mainstream.
Some of the topics work with each other (they could follow a casual chain), or they can be placed in emphatic order (least impactive to most impactive) to leave the reader with the strongest argument.