By long term I assume you don't mean in galactic terms, as inevitabley we're going to die out as a species. There's too much going against us - sun going to die, nearest galaxy on collision course etc.
In the short term population control would be a lot more effective than colonising Mars. I for one don't want to be eatting Soylent Green.
When are we going to realise as a species that having more than 2 children isn't sustainable - will there be a tipping point? Perhaps in the harsher parts of the world, for example Sudan is claiming 90% of Egypt's water supply (i.e the Nile), and now say that's not enough, they want 95%. First signs of a tipping point? Perhaps the foot shortage scare that got briefly put into the news last year.
All signs, but I think more likely we'll simply put up with it due to human nature.
And for what, because people have the right to have a third child? Silly. Very silly.
Edit: That said I'd really like the species to go to Mars. If you know you're going to die out one day, it should be on the todo list.