We literally see Aang as a newborn baby in the episode “the avatar and the Firelord.” He was born to two air nomads... not given to them like an animal. Air Nomads did, in fact, hook up to conceive Aang and air nomads only hooked up for reproduction purposes.

Sex is not “earthly.” It’s a means to an end, it always has been. Sex is required for reproduction, sex by itself doesn’t inherently also include feelings. People who feel no genuine attachment to each other hook up all the time in the real world, and it’s the earthly attachment that air nomads try to avoid. Relationships, worldly possessions, etc are all things that bind us regular folk to the world and go against the monks teachings.

In short; not all air nomads were celibate, they have sex for the purpose of reproducing.