Making 'redneck' jokes about incest and the like totally goes against the facts that it happens everywhere and isnt just a 'rural backcountry' thing.
This may be akin to claiming that polygamy "happens everywhere" and is just as prevalent in, say, NYC or LA as it is in Utah. In a country as large and as diverse regionally as the USA is, there are definitely cultural practices that vary significantly from one part of the country to another.
ETA: The median age of first marriage by a woman in Utah is only 23.3 (by contrast, the median age of first marriage by a woman in Massachusetts is 28.6). Now, this says nothing about polygamy or incest rates, but it does illustrate that there can be vast cultural differences between different parts of the country.