The avians’ more distant ancestors laid their squishy shell-less eggs in freshwater. But then the world underwent a long warm period, the oceans expanded, and freshwater became rare. The solution that evolved was for the other sex to carry the eggs in a marsupial-like pouch. Give or take a few million years later, avian’s direct ancestors were still using this method, and usually mating by both hanging from the same tree branch, facing each other. Eventually they lost their gripping wing hands for feet, and the sex position shifted to standing, although some cultures prefer to mate laying down or other alternate positions.
Brights develop eggs annually, usually in the spring or early into the wet season in equatorial climates. Coitus involves penetrating the muscular pouch opening with the ovipostor, and after some rubbing and rocking, a ring shaped piece of tissue near the tip of member becomes erect with lymph fluid. This O-ring locks the end of the ovipositor behind the muscles of the pouch opening and keeps the channel opening to transfer several hundred small soft-bodied eggs from the oviduct receptacles of the bright to the dun’s pouch.
After laying those eggs in a pouch, the opening closes, one egg is fertilized, and the rest absorbed for material by the growing child. The vast majority of physical material for development is provided by the bright, but a dun’s pouch will secrete thin nutritional fluid loaded with beneficial microbiota and hormones to help the embryo develop. Brights may make up to three clutches per mating season depending on age, health, and food availability, but duns can only successfully bring one child to term per pregnancy. Twins are almost always stillborn prematurely because of the limited resources they start with.
Common birth control methods relax the pouch sphincter or deliberately trigger natural abortion. Homosexual activities are usually manual or toy based, cloacal sex is rare because the cloaca is uncomfortably tight to most ovipositors, and oral sex is relatively rare outside of toy manipulation or sadomasochistic contexts. Conservative cultures view non-procreative sex with brights and dun birth control as exceptionally wasteful due to the limited opportunities for procreative sex during the mating season.
Outside of the 1-2 month period of the mating season, avians are generally uninterested in sex, romance, or intimate physical activities. They tend to view the other sophonts as hypersexual, transposing their perception of sexual culture in spring onto other sophonts. The reality is that avians in spring aren’t any more horny than the average adult sophont is normally, there is just a greater sense of urgency to act on the feeling while it is available. The excitement is also lubricated by the species-wide “vacation,” spring events, romantic holidays, and parties that happen that time of year.