That's ridiculous. There are many women out there who are proud of their marital status. It's a big deal in some parts of the country.
There are better places to fight your war on American traditions than your cart checkout. In fact, I would argue that the carts need more choices. For a good example, check out http://www.bodenusa.com. They offer a big list of salutation options, including awesomeness like "Baroness", "Colonel Sir", "Lady", and "The Honorable". If I ever get a piece of junk mail addressed to "Field Marshal Lord Snell", I'll know who sold my information.