If a woman goes out and sells her body for sex, it's likely either because she couldn't get a normal job and is desperate (in which case you'll be punishing some woman down on her luck just for trying to earn a living), or she legitimately enjoys doing it as a job (some people would love to get paid for having sex).

The only issue is spread of STDs. But if you view it as just another job, you could have proper healthcare for sex workers and testing of clients to ensure STDs aren't spread around. I hear people who act in porn get tested regularly, I don't see why prostitutes shouldn't.