Your personal anxiety and insecurities are what are driving your inner psychological fears. relax!
As for the safety of Amsterdam, it is very safe. I visited, and had a blast. They are required by law to have protection for any sexual act. One girl showed me the book of laws on it and it was nearly a half inch thick!
Sex workers in Amsterdam are not required by law to have HIV/STD check-ups, but most of them do voluntarily. Also, nearly every licensed sex worker will not do anything without a condom - so the risk of them having an STD is very small. If you use protection the whole time, the chances you will catch anything are extremely small (practically zero).
Have fun!!