There is ALWAYS bacteria in that area that isn't in any other parts of the body such as e.coli and if for some reason you get sick or your immune system is weak, it can cause problems for you because the anus wasn't made for licking. That is why it's closed tight and between to basketballs.
You can still do it just get some non-latex dental dam, put some lube around the area for full sensations and lick away.
Hot Dam Non-Latex Dental Dams Only $1.95/1-Pack