"An unmanned rocket-load of supplies, together with a factory to make fuel for the return trop, would be sent in advance. The crew would then follow -- heading straight for Mars, rather than using the huge and expensive International Space Station as a staging-post."

Of course fails to mention that it would require a new booster at least twice the payload capacity as the existing largest boosters AND Nuclear reactor engine- or simply a much bigger chemical booster, say about 8 times the payload capacity of current ones. With this additional capacity you would be in the neighborhood of the possible.
It should be noted that the "family" of heavy boosters are presently "requiring" govt "assistance" of the tune of a billion. Some space analysts have been critical of the Miltary's choice of funding the development by two lanch companies (Boeing and Lockheed) because of the lack of demand (creating an over supply of large launch vehicles). Making a additional larger booster would make this even more of a problem. And/or having NASA build and operate such a vehicle would be a complete nightmare of the private launch market and waste billions in development costs. This "nightmare" would would be in terms of added cost to the govt and added cost to satellite makers. It would be a repeat of the Shuttle- destroying the America launch market (which helped bring about the birth of European launch market- which still dominates the market- [or you could possibly destroy the European launch market too {possibly facing international lawsuits for "dumping" or have more restrictions on trade}. And the launch cost reduction will have seen over the years has been largely due to the European launchers, certainly not by anything NASA has done with the billions it has spent.
Now, had NASA started a lunar program to explore for water, the govt not be current in the position of "needing" to bail out this companies. They could have instead have bought about 1-2 billion dollars worth in launches AND possibly provide a future large market for these vehicles. With a large enough future market *perhaps* "better & bigger" launchers could be developed. Or perhaps simply a lower cost per lb *might* have been achieved (you would built larger boosters to lower costs per lb but depending on demand it might be better to simply increase production efficiency rather than than pay for additional development costs).