The problem is that your notion of "real problems" is artificial. If our society prioritizes space colonization above eliminating nuclear stockpiles, and people are willing to accept the risk posed by the existence of those stockpiles, then there is no sense in continuing to refer to it as a "main issue facing our society". Maybe you consider it to be a main issue, but society doesn't. Or maybe society does consider it an important issue, but not so important that everybody should stop working on everything else until it is solved (and as if Earth won't have new, seemingly equally important problems, when that happens). At least the people who are excited about space colonization don't propose to start dealing with environmental problems only after we've colonized Titan. You can't pretend to care about society when you are willing to ignore the actual things people find important.
Your fallacy is the same as claiming that we should care more about car accidents than about terrorism, because car accidents kill more people, instead of realizing that the number of dead simply isn't the only thing people care about when considering threats of this kind.
It's also unclear to me why you choose to single out space colonization when the vast majority of what most people do on Earth has nothing to do with solving the problems you mention. If anything, working on enabling space colonization is far more beneficial to life on this planet than almost every other subject on HN.