Define sanitary.

No body to body contact is truly sanitary in the strictest sense.

And, you're absolutely right that the proximity of the anus to the genitalia means there will be at least occasional stray bacteria reaching said genitalia.

However, because the non-STD bacteria that are the biggest risk with eating ass aren't as well equipped to last on the balls or vulva, and the vagina itself is moderately acidic, the risk of e-coli getting into your mouth in enough quantity and being alive enough to get past your immune system is lower than with direct analingus/rimming.

The butt cheeks are also a much lower risk, but all of the area is a non-zero risk without serious efforts akin to surgical scrubbing. Even that is only temporary.

E-coli can indeed be found in the vagina, on the vulvae, and on the scrotum. That's one of the reasons that you should always use a barrier during sexual contact. Even if you know someone is STD free, you have to be prepared for the inevitable cross contamination. If you're with a partner long term, that's inevitable, so you might as well just get it over with, but for short term or single encounter partners? Dental dams and condoms are cheaper than antibiotics.