Does it really matter? I mean honestly... until we discover other life out there, nothing we do matters one little bit as long as we stay alive. We owe it to the future of our species to diversify as much as possible. Even if we go green and save the planet, our planet will end anyway. Accidents, asteroids, etc. Something will get the planet, it will.

But even if we go farm out every other planet and make it barren and even if we mined every ounce of every other planet in our galaxy... if we're the only life in the area, or the only life in the universe, it doesn't matter one bit. We could blow up every single other planet and it wouldn't hurt us at all, because we can't live there.

So the only thing that makes sense is to keep living like we are the only life in the universe, all the while trying to figure out if that's true or not. Mars isn't alive. Venus isn't alive. The moon isn't alive. It's not "reaping" a "host" any more than me painting the walls of my house is. It doesn't make one lick of difference. There's nothing immoral about spreading to other lifeless planets.