My very immature brain just goes to "but poo comes from there".
Honestly though, I know far too much about the sicknesses you can get from poo (culinary background), and I am in no way, shape or form willing to stick my tongue near a place that could give me those icks.
In theory if my husband wanted it, I'd have him shower and we'd use a dental dam in place. Personally, I do not ever want it done to me though. Tongues near my asshole will 100% take me mentally out of the situation.
Fully support all my booty eating baddies, I'm just not one of them.