[gizmo686]

>The main reason is that we simply do not know how to sustain human life, in the long term, in less than 1 Earth gravity without deleterious effects, including the possible cessation of reproduction.

We do not know when the difficulty starts. Given our current knowledge, it is entirely possible that Martian gravity is sufficient for human health. We also know that even 0G is not acutely lethal (although the short term effects of returning to normal gravity are less then encouraging if we expect the people to be able to work as soon as they land on Mars).

This is just to say, we do not know how much of a problem Martian gravity is. There are still all of the difficulties we do know about.

It will probably be a long time (if ever) before we get a sustained "colony" on Mars, but humans setting foot on Mars is reasonably possible in the relatively near term. If we really wanted to, we could even make a "colony" simmilar to the ISS, where we have a permanantly staffed base, with a crew that stays for "short" (~year) time on Mars, combined with a year or two of total spaceflight.

I'm not remotely convinced this is worth doing. But it seems possible if we really wanted to do it.