Having a family is partially a financial decision. People should make the decision with eyes wide open, having planned for it. Achieving a financial position above sustenance before having the expensive family is generally a good idea. Same as buying the house and cars.
You can have a family, a house be sustainable but not earning enough to be able to pay for a year off of work. Which is the lot of 99.99% of people on this planet.
I think we have different definitions of "sustainable", then.
What you're describing sounds one step up from living paycheck to paycheck. And the fact that "most people are in that position!" doesn't make it a good position to be in, or a necessary one.
But, you can go on road trips, go camping, you can take a cruise, or find an all inclusive resorts. You won't have as much time for yourself like you did in your youth, but you can still travel and you'll make memories with your family, and show them new things about the world.