Why is incest so popular?? What is the appeal of it??

[pastispresent]

I avoid incest in general whether stories or threads which have that name. It makes me feel discomfort. I don't want to judge anything or anyone. 

I am curious to know what is the appeal of incest?? I mean not only in sites like xossip/xossipy but even on porn sites, incest is such a popular theme these days. It just turns me off sometimes......