That entirely depends on the income level you're talking, which is the OP's point. If you're making $10 mil and spending $7.5 mil on housing, you probably already have enough rooms to have kids. And even if you didn't, you could surely find room in your remaining $2.5 mil to add a room onto your house or find a slightly bigger place. This obviously does not apply if you're making $50k... Hence why absolutes are relevant.
You are assuming that 7.5 million buys a multi room home. At the extreme it may only rent you a 350 square foot apartment.
Some people might spend 75% of their income on massive dwellings, but most people downsize if possible before going past 55% of income. And as we are talking about things on the social not individual level those preferences matter.
