Colonizing deep space is a good idea, but colonizing Mars or any other planet is Stupid. Exploring and scientific surveys of planets are good but living there in the billions is stupid.
A rotating space habitat can be made to have exact Earth's gravity to which we have evolved and which we find comfortable. Mars or any other planet will never have the exact gravity of Earth.
A space habitat can be made to have the exact atmosphere of Earth. To change the atmosphere of Mars or other planets is impossible because it does not have the Earth's gravity to hold any altered atmosphere.
A space habitat can be made to have the exact temperature that we want. To change the temperature of a planet is extremely hard.
A rotating space habitat with exact Earth's gravity is not a gravity well that you have to climb out of. Move a few metres outside the habitat and you have zero gravity immediately. Whereas by stranding yourself inside the gravity well of another planet you restrict space travel as you have to climb out of the well every time.
A space habitat can support at most millions but mostly in the thousands range. Thus you will have lots of space habitats to accomodate billions. And these space habitats can be made mobile so they can distribute throughout the solar system. The loss of a single habitat to an asteroid impact will lose only thousands and even that can be avoided by moving the habitat out of the path. A planet on the other hand will put billions in one basket so a single comet strike can take out billions. A planet is impossible to move.
A single moon or planet can be melted to form trillions of space habitats, supporting human population in the quadrillions in a single star system. Again a single star system is one giant basket so we have to spread to other star systems and galaxies as well.
Even if we find other habitable worlds in other star systems, they could already have an ecosystem and perhaps even intelligent life. If we learn to live in space we can colonise such star systems without destroying indigenous life and ecosystems.