It has occurred to me recently that there is a big difference between allowing detached "single-family" homes and requiring them. New urbanists such as myself emphasize the value of denser living arrangements, because they save land, promote forms of travel besides driving and so forth. Does that make single-family homes bad? No, I don't think so. Sometimes land is cheap and there isn't much need to develop it at higher densities to make it affordable to people. Later, maybe decades later, the land may become more expensive and then dividing that expensive land over many units (e.g. through bungalow courts, townhomes or apartments) can be a good way to ensure that more people can afford to live in the area.
Zoning regulations that allow nothing but single-family homes are problematic. These regulations, which are found in most American cities, are used to prevent property owners who want to build more densely from doing so. This is a major reason why housing is so expensive in so many cities. The California Legislative Analyst's Office has pointed to this housing shortage as the primary driver of the state's housing affordability crisis.
What we have is a system where neighbors have a lot of say over what property owners can do with their land, but property owners don't have much control over their own land. This has led do some interesting alliances, as shown in this libertarian video on the progressive activists leading San Francisco's "yes in my back yard" (YIMBY) movement.
Building more housing in existing urban areas has a lot of benefits. It's better for the environment than building low-density sprawl (by saving habitat and making people less reliant on driving). It makes areas more accessible to the poor and people of color. It means infrastructure is used more efficiently and places a lower cost burden on taxpayers. I've made my peace with single-family homes. They're not my personal preference (I prefer walkable urbanism, and the denser forms of housing that support it). However, they are okay, as long as you don't ban everything else. Live and let live.