To be totally fair, a sphincter (I'm assuming this is what you're referring to) refers to a circular muscle structure in the general case. Not just the butthole.
There are sphincters in your eyes.
There is a sphincter in your esophagus.
Your body is full of sphincters.
The anal sphincter is just one of many sphincters.
That's irrelevant, because the association is ever-present.
As an example, if people lived on Venus, they would be called "Venerials" by the proper genitive form. Alas, doctors got to it first (Venus, Roman goddess of love...) so it was changed to "Venutians". (Source: Neil DeGrasse Tyson's podcast, StarTalk radio).
I'm sorry, but this cracked me up. It brought out my inner Beavis and Butthead. I apologize. But, I mean, come on. Most people do associate the word sphincter with the butthole, and surely the creator should have known that. (Unless the, umm, cheeky association is intentional?)
