The thing is, Profesco, we can't just rely only on our intrinsic instincts. I find homosexuality extremely disgusting but I'm not going to say nobody absolutely should do it.
Same thing with incest. I go by this policy that if two parties are consent and avoid having any non-consenting relations involved (i guess this would be the result of it or doing it in public, etc), then I see no problem with it from a neutral standpoint. We're not going to do anything about it, anyway. I just find it dumb to patrol people's bedrooms. You're not going to find out about it, but the least you could do is tell them to be safe with it.
Besides, there is worse out there than incest. What about BDSM? There are people out there who enjoy pain so much that they are actually inflicted heavily. There are people who probably do dangerous things that are fatal, but once again, patrolling the bedroom is just unrealistic. I refuse to go by my 'normal' instincts on every single issue, because my normal instincts either aren't always right or always logical.