It is not entirely clear whether being circumcised can reduce your risk of aquirring a sexual transmitted infection (STI). There have been several studies into male circumcision and the risk of STIs, but the results have proved inconclusive.
What has been observed is that men who have been circumcised are less likely to notice the symptoms of STIs such as chlamydia, which in turn heightens the risk of them passing it on to unsuspecting partners. Circumcised males also appear more likely to develop penile warts.
There is also conflicting advice with regards to male circumcision and HIV/AIDS. There have been several trials carried out in Africa that suggested that circumcised men have a lower risk of acquiring HIV from infected women, however the international not-for-profit health organisation the Cochrane Collaboration reviewed all the research into circumcision and HIV and concluded that there is insufficient evidence to support the idea that circumcised men have less chance of contracting HIV. To date, there is no solid conclusion either way.
The only way to effectively protect against sexually transmitted infections and diseases, including HIV and AIDS, is by wearing a condom during sexual intercourse.