Interesting point was raised while talking about Wudu with a non Muslim friend...she understood why we need to make Wudu after we have been to the loo etc, but not for ahem.....farting...I couldn't really answer it as well, if you aren't following through each time you fart(which I sincerely hope none of us are!!) why do we need to wash again as its just gas really....
It is a tad indelicate but an interesting point..