Most people condemn incest because it is a social taboo. A sexual relationship between brother and sister, for example, is not usually seen as right because the relationship between brother and sister is stereotypically much different between the relationship between lovers, so people find it hard to comprehend the crossover.

Incest is, in my opinion, very wrong when children are born. Defects are very common in these relationships due to similar DNA. Also, it gets very messy when an confused underage sister/brother is bought into the equation. Family units are ruined and put under public scrutiny.

The recent case of Elizabeth Fritzl was incest, but not forced rape. As far as I understand, her father manipulated her into consentual sex, due to him being an authority figure to her. That is when incest is very wrong. A parent/child relationship is a huge taboo and usually involves high levels of manipulation.