This is an odd one, as incest has such negative connotations, but as far as I can tell the only reason against it is the genetic risks to any resulting children from the inbreeding. There are cases of incestuous abuse, but if both parties are consenting this doesn’t really apply. The actual abuse is the issue in such cases, rather than the incest per se.
For the record and before the torch and pitchfork brigade arrive, I’d just like to clarify from the start that I have no incestuous leanings myself: This is simply about attacking what appears to be an unfounded prejudice!
Incest is taboo in virtually every society, but if we compare it with, say, homesexuality, presumably the parties involved can’t help feeling that way. In fact, genetic sexual attraction is a very real factor – ever noticed the similarities of the faces and features of couples in the marriage announcements? If having kids was removed from the incestuous relationship, what is the problem, say, if the male/female was sterilised for instance? For that matter, if the relationship was non-sexual as with non-practicing homosexuals, would there still be a problem?
My concern is that when people consider the concept of incest they immediately gross themselves out by thinking of how much they would NOT go for their relatives, and this disgust becomes associated with incest in general. For myself, I love my family dearly and have no desire to have sex with them, but I cannot see any reason that incest should be declared taboo for responsible consenting adults (dealing with the genetic risks) without appeal to some religious mandate. The religious standpoints vary in terms and conditions of permissiveness and the nature of the blood relationship, but I cannot find any justification. Given the number of agnostics and atheists, why does the taboo persist?