I think they found out when studying maternal vaginal swabbing for seeding microbiota Caesarean newborns that the bacteria were actually either airborne from the maternal fecal route or somehow getting into the baby from there

Pooping during delivery is a thing. So that's entirely believable.

I don't think licking butts would do it. The gastric acids in the stomach might kill most of the good stuff?

Now a turkey baster with a partner whom has no STD's might be doable.

I once had a perpetually happy virginal, before me, girlfriend whom would have made a perfect donor.

“Your sexual partners can change your microbiome, a new study in mice finds“

https://www.statnews.com/2019/04/05/your-sexual-partners-can...