Why not? There are plenty of fetishes that are as weird or even weirder.
Why are people into scat and/or golden showers? Being choked? Being hurt? Being humiliated? It's all about the taboo-ness of it.
I have plenty of cousins that I would not have minded hooking up with. I actually almost did, once, when visiting my dad's side of the family when his dad got sick and was in the hospital. Things got a little flirty, but nothing much further than that, 'cause we were only there for the day.
And never forget that most US states allow up to 3rd cousins marrying, and some allow up to 2nd. (You know who you are. Perverts.)