If you sit...your ass cheeks are spread out on the toilet seat for easy access...when you stand, they are snapped shut, unless of course you bend over...then all you have to worry about is getting the toilet paper in the toilet and not on the floor...common sense
How would one sample a Spanish vagina? Do we find some Spanish chick on the bus and hold her down while we shove a rather large cotton swab in her pussy and then send it off to a lab somewhere to be tested for pathogens? Or do we just attempt to carefully cut the whole thing off? Now I'm sure the other passengers wouldn't be to pleased about this - what with the blood and the blood-curdling screams in sheer terror and pain and what not. But hey, fuck those cunts, science is forever changing and if those bastards can't change with the times then they should be doomed to stay in whatever hell their current lives are in science free!
To this day, that part in the movie really stumped me. In the 17 years since that movie was released I have tried again and again to work out a logical scenario in which 3 seashells could aid someone in cleaning their ass. The only way I can work out that even makes a tiny bit of sense is that you use two of the seashells to spread your ass so that the shit doesn't get on your ass and there's nothing to wipe. But even that half-idiotic (maybe more than just half-idiotic) explanation leaves the 3rd seashell out of the equation...
Long story short: You are not alone in recognizing the reference.