The real answer wrote:
They don't decide anything. They already have a life and they just keep living it. Having kids just isn't a priority for a lot of couples and there's typically no need to decide otherwise.
On a related note, kids are seen as giving a couple's life meaning in large part because most working adults don't have any major purpose to their lives outside of work and paying the bills. Eventually, the nights out and occasional vacations get old, and they peter principle head-first into the ceiling of their careers. Suddenly, having a kid gives you a permanent commitment and a new purpose. If your life prior wasn't anything to write home about, then raising a child quickly becomes your most meaningful commitment.
It is expensive to have kids, and have day care, etc. A lot of people I know who have kids in the Cbus area (in both run clubs and Irish Dance community) are upper-middle-class and work at big Cbus industries such as Nationwide, NCH, Huntington, Safelite, Chase, etc. They are also middle-managers in their late 30s to mid 50s. Families that are younger either only have 1 kid or are struggling and living near Whitehall.