Firstly, a nice demolition of the idea that a one-way mission to Mars makes sense, written by Oliver Morton, whose excellent book I reviewed. The best concise point:
Most importantly, in terms of costs, there’s the ongoing commitment. A set of Mars missions you can cancel is a much more attractive than a set of Mars missions that you cannot cancel without killing people (“Launch the next rocket or the kid gets it”). To fund a single one way to Mars mission is more or less to sign up to funding them for as long as the colony lasts. That is a far larger spending commitment than required for a small number of return trips.
I certainly wouldn’t want to be one of the decision-makers responsible for keeping a Martian colony alive, while billions are watching via high-definition video links. Watching the astronauts slowly (or quickly) die would be awfully depressing, after all, especially if it was because of budget cuts.
Secondly, a Science article on the importance of not contaminating Mars with terrestrial organisms: Biologically Reversible Exploration. In essence, it argues that contamination from terrestrial spacecraft could forever eliminate our chances of studying life that evolved independently on Mars, if any such organisms exist. It argues that future missions, including any manned missions, adopt protocols so as to be ‘biologically reversible.’ As countless examples of terrestrial invasive species demonstrate, the concerns are not unwarranted, when it comes to microorganisms that might be able to survive or thrive in the Martian environment.