In all seriousness, at face value, I agree — consenting adults should be able to do whatever they want. However, things get sticky when you introduce reproduction to the mix.
Charles Darwin was one of the first scientists to demonstrate the effects of inbreeding depression, through numerous experiments on plants. Darwin's wife, Emma, was his first cousin, and he was concerned about the impact of inbreeding on his ten children, three of whom died at age ten or younger; three others had childless long-term marriages.[14][15][16]
If this was happening where I lived, I would care about the potential impacts on the community from the possibility of an increase in genetic disorders of children born from such a union. Including increased stress on Healthcare, increased mortality rates during childbirth, increased citizens on government assistance, etc. Not saying these things would happen, but I would care enough to look into it.
There's also the impact on reputation resulting from one of the states leaders thinking this was an important enough issue to create a bill for, and then the impact on reputation if it gets enough votes to pass. Would the opinions surrounding morality and / or reproductive rights carry less weight? Could it even sway people away from the side taken by Kentucky just because it is the side Kentucky is on?
Well keep in mind the odds of a birth defects from first cousins is the same as a women over 40 have kids at all. But i don't see applying that argument to this case.
You say that, but it's still really rare in most countries with only about 10% in the entire world. It seems that Arab countries are typically the exception and are responsible for most of that 10%. There is probably a lot of cultural nuance there that I'm missing, but the link below has a world map that really illustrates that it occurs mostly in the Middle East.
I did some searching and you're right that it is practiced in high amounts in the middle east however you also find the same in North Africa and parts of Asia (specifically the Indian subcontinent, parts of India and southeast Asia). You also see its practiced in smaller amounts around the world. Including Europe and South America as well some countries to a lesser extent in North America. I'll provide one source below but you can search for more.