What is odd is humans seem to be the only species that have a sense of disgust in the first place, sometimes like in this case it can be overactive.
A dog for instance wouldn't even have the slightest issue drinking out of a toilet bowl, it would never understand why it was disgusting to us and it most definitely wouldn't gag, in fact I don't think any other animals would gag at anything we would find disgusting.
I constantly clean the toilet here... I also try and keep the lid closed (but I'm a guy and sometimes I forget)... The reason for my madness is just what you stated...
Mia (my dog) will have a clean bowl of water, but if she gets the chance she will go drink out of the toilet... Plus like any dog, she licks herself... I don't know why she does this, she is clean all the time? The noise she makes while performing this task, alone makes me sort of queazy. And If she licks my face its all coming up... I even brush here teeth and she gets mad and sulks over it...
I do not know what has always made me have this mental, visual, smell thing that causes this to be so extreme. Its funny most the time, but also horribly embarrassing at the same time.