That's pretty much it in a nutshell.
I know what you mean about there being lots of "me" (ooh, there's a thought... NO! Stop this vanity! :oD)
But my thought is that as Stonehenge, and many, many other sites like it, have been there all this time, with people sittling, climbing, etc all over it, then is it such a bad thing? Maybe Avebury is not the best example, but what is available there is fully interactive, and looked after to a certain degree. I know there have been several acts of vandalism to the site, but as it is, what, 5000 years old(?) it's not done too badly! I'd expect a bit of wear and tear!
A lot of the sites I've been to, mainly in Cornwall, are not fenced off, but get plenty of viitors, and are a joy to be in amongst. Not to be too much of a hippy (again - not such a bad thing..!) but I do like to touch, to get a feel of the place. I can see the Stonehenge that I saw the other day on telly or on the internet.
I'm sure it's not quite as clear cut as my simplistic view, but it would be nice to think so!