Argument: Incest frequently involves abuse of power in family relationships
"Should Incest Be Legal? The Limits of Privacy." Philosopher Dad. April 9, 2007 - the reason that incest differs from other types of consenting sex, is the underlying power dynamic that exists in families.
Parents exert strong influence on their children, even once those children become adults. This is akin to, but more powerful than, teacher-student relationships. The child may not even be aware that they are being coerced into sex.
The privacy arguments that have been used to strike down laws against sodomy, simply do not apply because the issue of consent is completely different.