I'm more concerned with Mars. Mars potentially had life, once. There's even a vague chance that it might still, somehow. Evidence for either would be extremely precarious. Unlike the Moon, every bit of waste vented into the atmosphere is going to be carried around the entire planet by the wind, tainting everything.
If it were up to me, I'd declare a moratorium on humans being on Mars. Not permanent, but, say, 100 years. Enough time for us to send lots of robots to explore the planet without mixing our biology up in it.
Build up whatever automated support is needed for that. It's industrial and ugly, and surely dumping out its own non-biological pollution, but that doesn't risk breeding the way the way our microorganisms do. And everywhere we go, we spread microorganisms, lots of them. (In theory they'd be contained, but accidents happen.)
I'd settle for 50 years. Or even 20 years past the point where we'd be capable of the kind of rockets required to build an industrial infrastructure. Just gather a crapload of data from the planet as pristinely as we can, just because we're never going to get another opportunity. Use that time to build whatever you want on the Moon.
Of course it's not up to me. There will surely be astronauts on Mars well before I think it's a good idea. It will certainly be exciting and produce a lot of great data fast. I'll just cross my fingers that they can get it while it's still there to get, before every test turns up "We found DNA on Mars... oops, it's us again."