The problem is that in the game of home ownership the situation where every couple buys a home that they can afford on the lower of their two incomes is not a N

The problem is that in the game of home ownership the situation where every couple buys a home that they can afford on the lower of their two incomes is not a Nash equilibrium. If everybody else is doing that, then you and your spouse can be the exception and use both your incomes to get a mansion. Now everybody else sees how well you're living and they mimic your approach. It doesn't take long until it is no longer possible for families to buy a house on the lesser of their incomes.