Well first off, if you are going to lick any asshole, you need to have that person clean it first (enema before anal sex is a good idea too). Simple soap and water should do the job during a shower.
The stimulation of the anus is natural, because if you look up the nerves in the area around the anus, they connect to the clitoris or penis. Tickling a partner's anus with one finger while preforming oral will also give positive results.
Licking an ass is as sanitary as giving head if cleaned prior to those actions.
With all oral sex, safety is key and knowing your partner well is important, STD's are bad...
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