Big asteroids are a valid concern, and very long-term I do believe humans should work at establishing a human presence on other worlds (starting with the Moon), however asteroid bombardment should *not* be a factor in driving humans to inhabit other worlds.
It would be far, far easier for us to improve our capabilities for detecting large asteroids, and then deflecting them, than to figure out how to live on Mars. Dealing with asteroids is not that hard: first we have to actually invest some resources into looking for the damn things. We do a little of that right now, but not nearly enough, as the strike in Russia a couple years ago proved. This isn't hard; we just need more probes in orbit, or perhaps in Solar orbit closer to the Sun (to spot ones that we can't see from here because the Sun's light drowns them out). Second, we need to develop the capability of deflecting them. With good enough detection, this isn't hard: you just send a big craft up there with some engines (probably ion engines) and a lot of fuel and run them for a long time to push it into a slightly different and safer orbit. If you have enough forewarning, it's not that hard, because a little movement will make a big change in trajectory over a long time. The key here is having enough forewarning; if your detection efforts are so lame that you have very little warning, then you're not going to be able to avert disaster.
Simply put, it'd be a lot easier and cheaper for us to invest in some space-based telescopes optimized for detecting Earth-crossing asteroids than to develop all the technology and infrastructure needed for establishing a colony on Mars. And the end result is better too: instead of some small colony on Mars surviving while the bulk of humanity perishes, along with the most livable planet for humans, we can keep our planet and the entire human race intact.
But if we're too stupid and short-sighted to invest in some telescopes, then maybe we deserve to be wiped out like the dinosaurs.