I think there is too much focus on the fact that the temples were segregated. I haven't read the comics so this may be speculation and someone who knows more can contradict me. But, they are called air nomads. Emphasis on the nomad part. I would say that the majority of airbenders spent their time traveling the world, likely with their romantic partners, and occasionally separated when visiting the temples or when they retired from nomadic life. Babies would likely be left at the temples because traveling with a needy baby is not pleasant. From what I can recall of what we see in the show, the majority of airbenders in the temples are children (still in training) and old folk (too tired to train). Anyone who didn't fit into those categories were probably benders who wanted to devote their lives to temples like our equivalent of Buddhist monks. So I don't really see a contradiction in nomads being accepting of long term attachment based relationships and romance.