Thanks for guide line......
Use a condom. This will help protect against any infections and STD's.
If the woman has AIDs or HIV that can spread via anal sex. HPV can also cause some problems, but usually men don't exhibit any symptoms of HPV and it causes no problems. On the rare occasion it does, you would get gentile warts. So unless you're 100% trusting of their STD status or have a report from the lab, best to use a condom.
Also it's possible to get some nasty infections under the foreskin so just make sure you're washing it well after sex.