>>42267920
I know people have already pointed out that Humans≠ponies but to sate your Victorian worldview for a moment think about it. We do see the siblings of ponies whose parents wanted to mitigate risk because both Earth Pony reps, Applejack & Pinkie do have multiple siblings as we see.
In fact Rainbow Dash is kind of an exception in being an only child among them. While the Earth ponies have three siblings each we see the Unicorn or Pegasi Mane Six with one sibling, meaning two children. Equalling a stable population. We see Unicorns occupying many upper class positions in Equestrian society, so we can understand there's less need for labour, less danger in their childbirth, so on.
Pegasi we know come from an original martial tradition, which would mean many children, but by the Mane Six's time we see the most popular jobs in Cloudsdale is the weather factory, or the service industry in Las Pegasus. Which once more gives them more urban amenities, less need for children than the overwhelmingly rural Earth Ponies.
