But generally, incest is bad because it magnifies a lot of issues of romantic relationships and lessens the integrity of family.
If anyone thinks incest is good, then please stop watching porn.
Also, incest can be quite identifiable, even at a young age. For instance, a girl is asked by a neighbor's child "Who's your boyfriend?" to which the girl replies, "My brother." Then the neighbor's child says "That's weird." Saying that an incest couple can not identify their own incest is like saying that a gay couple can not identify if they are gay. In fact, incest is a sexual relationship that many societies openly outlaw and ridicule, so it may be even quicker for an incestuous couple to realize that they are comitting incest than for a gay couple to realize that they are gay. The only few ways, to keep the incest couple "dumb," would be to keep them away from certain societies, which can be very difficult to do, especially with the internet. It's more likely that an incest couple will feign ignorance, to avoid being convicted of incest.
Sorry if my tone is a bit frustrated here, but it's disturbing to think that some people would defend incest and then reference history rather than current societies, observations, laws, and human behavior. Some people in this thread would not have the patience to teach that incest is bad and to quit porn. But I suppose this is a debate forum, after all. And I've been misguided before, especially through porn, so I empathize with the confusion.