By definition, if you cannot live sustainably, you cannot sustainably live. Though sustainability, more or less, is about sustaining current populations and quality of life; most failures of sustainability won't extinct the human race, they'll just kill a lot of people off and deprive us of the massive amounts of energy needed to maintain our present quality of life.
If the definition of sustainability that you are using is "sustainably living" then it isn't [insert ideas associated with the sustainability movement here]. Not automatically, anyway.
If all we care about is continuing to live (excepting the limitations imposed by entropy) then continuously importing resources from somewhere else is a perfectly valid solution. For that matter, no amount of efficiency will ever eliminate the necessity of importing resources.
Pushing that necessity into the future is probably a good idea, of course. Continuing to live is not all I care about. But mature space infrastructure would positively contribute to doing just that, even if a colony on Mars might not in and of itself.
Whenever I hear that argument I want to scream. The logic flaw is so clear:
It is only unsustainable at current technology levels.
The way we lived in 1900 was unsustainable then, but we could survive like that with 2012 tech no issue (less coal burning, improved sanitation, etc). In 100 years it will be sustainable to live like we do today.
