Every normal human being on this meaningless floating rock in space is against incest on principle, and for good fucking reason too.
*sigh* When I say I think being against incest on principle is silly, I obviously don't mean that people should start treating it as a TOTALLY NORMAL THING that EVERYONE SHOULD BE DOING YOU GAIS. On account of not being an oblivious idiot, I'm perfectly aware that there are biological issues with the offspring of such relationships, and I think inherent biological tendencies (as well as social norms on top of that) are more than enough to ensure that such relationships are always going to be an incredibly rare occurrence.
I just think that in the unlikely event that they
do occur, it's silly to treat such relationships as horrible and offensive on a personal level. Unless the couple in question are planning to have children (and I'd like to think most people are sane enough to not go down that route), there's really nothing fundamentally wrong with it if that's what both parties want.
To bring up an awkward analogy (because doesn't everybody just love those), you could even say that hating on incest is essentially the same as homophobia. Both are a case of dismissing the validity of a relationship because it's a) not in line with most people's inherent sexual preferences (most people aren't attracted to the same sex/close relatives), and b) biologically illogical because it's not going to produce viable offspring.