Far fewer people are partnered, so that alone doesn't mean there is sufficient housing. Divorce/not marrying probably means we need 30% more bedrooms than previously required.

The average household size has not decreased significantly in the last 40 years (from 2.75 to 2.5), but units per capita has increased (currently around 0.425 units per person). We went from around 1.5% unit surplus to 6% unit surplus during that time.

You do bring up a good point, however. Nuclear families use housing much more efficiently than singles. For the good of the planet and others, perhaps people should consider the benefits of committed relationships.