I personally see incest as just an observational fantasy, or something to roleplay. My interest in it sits in that I had a really poor relationship with my father, so the idea of having a relationship with one is kind of a turn on. I by no means would ever want to fuck my dad, or my uncle, but as a situational thing with someone else I find it kind of sexy.
I also think it's sexy for a lot of people because it's such a terrible taboo. I mean, in the long run of things, mutual incestuous relationships are usually pretty harmless (at least, same sex ones - het ones can lead to genetic mutations) but it's still considered wrong, and to some people that's sexy, in the same way that having sex in a public place, or streaking is exhilarating - it's frowned upon, so it's exciting. I personally understand that it is wrong, and that it's not something that should legally be practiced, but that doesn't stop me being turned on about it.
but w/e, about 50% of these pictures are crappy anyway, so it's not a big deal.