where those screwy ideas come from is interesting to note but not that important. (for example there's a saying in latin american spanish that goes, 'el santo ni folla, ni mea, ni caga' which translates roughly as 'saints don't f**k, piss, or shit'. and certainly there's something in our christian heritage that deems bodily functions as somehow lower or baser, and therefore further from the Divine. it seeped into our language in sayings like 'cleanliness is next to godliness' (rather than my preferred version which is 'cleanliness is next to OCD'). and though sayings like these are on the wane and gradually fading out, we are all still prone to our common heritage - for better ...and for worse. especially if we don't examine and challenge what's been passed down to us.)
as an exercise, next time you're in a situation where you know full well that someone is in the bathroom pooping, try replacing your thoughts of disgust or whatever they are with something deliberately opposite, for example, 'good for them, i'm glad they're having a crap - hope it's a good one!'