My family lives in Portland, OR, which isn't nearly as expensive as (say) SF or NYC but has seen significant housing cost increases in the last 5-10 years. My wife works as a transportation consultant, which is a solidly white-collar/"knowledge" gig but pays far below what most tech gigs do. We could probably afford our essential costs (mortgage, taxes, car, insurance, etc.) on her income but we wouldn't have been able to buy a house here without the down payment that my technical work allowed us to pay, or the cash gift her parents gave us for our first (USDA-backed, 3.5%-down) home purchase.

My strong feeling is that most places with a robust employment market price most single-income households out b/c the expectation is now that you at _least_ have two earners. Kids take another chunk out of your "competitiveness" because of childcare, cost-of-living, and increased demands on your time (making it harder to really "excel" at work).