It depends on where you live. Where I am, a 4 bedroom house in the city (mediocre school district) is $180k. So $120k is a good chunk of change, and you can comfortably live on a single income.
If her vision of a "middle class lifestyle" is buying lots of shiny things, you're screwed.
Wow, 180k for a 4BR house!? Houses in the poorer suburbs of my city (the poorest capital in Australia) are still $400-500k +. Rent for a 4BR is about 450+ a week.
If her vision of a "middle class lifestyle" is buying lots of shiny things, you're screwed.