The technologies of the 20th century, much less the 21st (we're already 1/7th of the way through that century), are enough to settle Antarctica. And there are some seriously, seriously overpopulated places on Earth now, and the land values in Manhattan and London are preposterous.
But nobody is even talking about colonizing Antarctica for lebensraum. Not even doing that some decade in the future, ever. Nobody in India is saying "man, when India hits 1.4 billion, we'll just have to move some folks to Antarctica".
Or the Gobi Desert. Or the central plains of the whole of Russia, where some millions of square miles are barely used for anything, not being quite productive enough for grain growing. Only a *little* technology, compared to Mars, would be needed to colonize land areas the size of all of Mars...and are right here at the bottom of a gravity well, with free air(!!).
As Charles Stross says, call me about space colonies when Antarctica is full.
The professor's point is not that humans lack adventurousness or industry; but they don't go doing much more than the one trip and plant-the-flag unless there's profit in it. There's no profit in Antarctica, the Gobi, or the Russian Steppes...and way, way less in Mars, still less beyond it.
It's not even about transportation costs; you could lower those until you could get a tonne to Mars as cheaply as to Antarctica and there would STILL be no space colony unless Mars had something to offer than the Gobi did not.