That's why suburbs are (relatively) full of kids, but dense urban areas look like vasectomy sanctuaries.

Not quite. Baby boomers are mostly explained by delayed births due to the Great Depression, and then by greatly improved living condition in the newly settled suburbs: https://ourworldindata.org/data-insights/the-baby-boom-saw-a...

But the current drop below the replacement rate is all due to increased density. If you look at the stats, the urbanization percent stayed flat. However, more and more people are moving into a smaller and smaller set of large cities.