The fact is that the vagina is about 10 times cleaner than the mouth!
It has its own self-regulatory system that keeps it in check every now and then.
The vagina is full of good bacteria known as lactobacilli that constantly fight infections and keep it in good condition.
Even vaginal discharge is not dirty. It is made of water, albumin, cells and mucin (an oily substance that gives it the slippery feel). Hence a discharge is not a toxic substance.
Normal vagina also has a slightly pungent smell which is perfectly normal.
Menstrual blood is also not unclean and does not contain any germs as may be speculated. It smells a bit metallic, but contact with menstrual blood isn’t harmful at all.
All these make the vagina one of the cleanest points in the body. Therefore do not introduce an unhygienic mouth to the vagina during oral sex.