JK muttered:

I don't disagree with anything you say, I grew up with animals and I
believe the developing immune system needs chalenge. But as with
anything, it's a question of degree. We have a small-ish city garden and
approximately 1/3 of it is bark chippings. These ares were literally
full of kittycrap. I don't mind the odd turd, but I want to deter the
daily use as a litter tray. That much cat poo does pose a health hazard,
and it also stinks. No-one should be trying to tell me to put up with
it, get over it etc, would you have wanted the neighbouring farmer to
stash *all* his cow manure in your garden?

Hard pave it then, cats won't crap anywhere they can't bury and you have
provided a giant litter tray for them with the bark chip. Even that needn't
be wasted, sounds like perfect compost material ;o)