While I believe the risk of UTI is too high going from ass to vagina, I have gone from my own ass to my own mouth many times without issue. It would be irresponsible to suggest it, however, being risk aware, ATM is an acceptable risk to me. Unlike ATV, which has little capacity to defend against infection, the oral route has much stronger defences against bacteria.
I consider the risk of an E-coli infection in a healthy individual (me), which typically is a day of vomiting and diarrhea, vs the reward (humiliation kink) and on days that I can accept the risk (begining of a weekend) I do. I also avoid such activities if I am immunocompromised, esp if I am already fighting a GI infection as a result of food poisoning, but even if I have a small cold, I do not accept the risk. I will say that over the last couple years I have gotten food poisoning several times after eating at mexican restaurants, however, I have ATM more often, and to my knowlege have yet to have an issue.
Once again, I can not advise others do it, however, for me, I have not had a problem. Just understand the risk, and avoid the activity if the risk is too high to accept.