Sex and the Microbiome
Sex provides the genetic diversity necessary to raise the odds against an onslaught of pathogenic marauders. It is may be that sex evolved for that purpose (among others).
However exciting courtship may be to its participants, it is also a communal ritual of microbiomes. Kissing is ubiquitous in human cultures; although in some it is more like sniffing. While swapping saliva risks contagion, that may be the point.
Humans carry various chronic viral colonies. Acquiring new viruses during pregnancy can harm a fetus. So, kissing well before the event allows infections from a father to settle in and lessen the odds of mishap. In this case, kissing is a form of self-vaccination.
Human sweat does not smell. The odor of armpits comes from microbe excretions after feeding on sweat. Armpits are fermentation crocks indicative of health by way of resident bacteria.
Women prefer men whose body odor suggests a complimentary immune system to their own. The scent of a man is not of the man, but of his microbiome.
Prolonged proximity among animal conspecifics is a communal rite for the microbes involved. Its more than just sharing parasites and ne’er-do-well prokaryotes.
Insect colonies share symbiotic microbes that ward off parasites. Some salamanders nest in groups to share bacteria that keep nefarious fungi at bay.
Primates that groom one another end up with similar microbiomes; so do people who live together.