uh...we aren't.
And if you only mean conscious life, (1) I'm not sure why that is a special consideration, but I am familiar with the discussions around this point; (2) it is more than possible that certain animals, like other primates, dolphins, whales, etc., have some degree of conscious experience, and it is also more than possible that we won't find anything more obvious than that on another planet.
Edit: Ah, I see. You were saying there is no life in space. I was thinking more along the lines of, "what about all the environmental damage we will do in order to travel to outer space." I just didn't realize that I should have included that concern in my initial comment, since for some reason I thought it would be obvious.
Sure we may cause environmental damage to get there, but I think that is far outweighed by the benefit of being a multi-planet species.
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.