[yamtaddle]

Goods have to be shipped farther to reach you. You have to drive more than you might if you lived in a dense city (or dense town, but the US doesn't really have such a thing—hell we barely even have dense cities, really) even without the commute. You require far more installed and maintained pavement to maintain your quality of life. Utility runs are longer, so, more ongoing maintenance costs, which means more fuel use and more use of materials (so, also more fuel, ultimately). And so on.

Less-dense housing strongly tends to be less-green than denser housing, commute or no commute. The differences are extreme enough that yeah, it's pretty close to being that cut and dry.