For most of the existence of human species (according to the scholarly consensus of anthropologists) we existed in bands of adults who would intermingle freely. Adolescent men would raid nearby tribes and kidnap their young women, which is the means by which genes were exchanged between tribes.

All the monogamy and licensing happened after agriculture and the great leap forward once tribes became big enough that infectious diseases were no longer contained through pure isolation. We see the misogynistic trends rise in late Hellenic periods and then Christianity cranked it up to eleven, so now we imagine even our migrant hunter-gatherer ancestors paired off.

As a note, during the middle ages, it was super important among aristocracy to assure ladies-in-waiting were virginal before they were wed, and then used purely as heir machines, but the serf class routinely banged like bunnies in springtime. And while frowned upon by the more piety-minded clergy, it was generally ignored because a) Child mortality was something awful and every kid that ever reached majority was to be celebrated, and b) The labor shortage was extreme everywhere. There was always way too much stuff to be done, and so every pair of hands was welcome, even when they were attacked to an idiot, a malformed hunchback, a ne’er-do-well or the bastard progeny of a mixed coupling.

Curiously, as we see in the birth of Mordred, pre-Christian European traditions included suspending adultery limitations during holidays, which happened at least once a season, sometimes twice. So even in societies where monogamy was the norm, there was a defined space for getting a bit on the side. (Useful when your partner was infertile.)

So yeah. Right in one.

Probably because a lot, if not the majority of LGBT couples are DINKs. Makes it a lot easier to focus on career/education when kids aren’t around.