In my experience anytime I have asked a lady if I can lick her asshole, Even if she seemed a little hesitant at first, if she let me do it, they almost never wanted me to stop doing it, I have had it done to me by women and it just feels very good. Let her know in advance of your desires, and she will be prepared. There is usually little to no odor from a womens anus with proper hygeine which everyone should have before sex anyway, and the little bit of odor that you might get is actually kind of pleasant to me at least. Oral on the asshole over time is a great way to relax and introduce her to the next phase. She will probably ask to be penetrated by the penis sooner than later! As far as infections or bacteria, I usually do one or the other each time we engage in sex. This time ass, next time pussy. If she's good with it then Viola! Celebrate all of a womens private parts with her!