Well, prostitution is a big no-no in Singapore, though like any other country there is a legalised red light district (which is also a popular place for good food at night). You're unlikely to run into any "working girls" in the city, and even if you do, chances are that she's not a Singaporean and perhaps a foreign worker. We're known for that clean image, ya know?

You'll find that the laws in Singapore are pretty strict, and the penalties tend to be harsher than the average asian country.