There are a lot of problems with many child pornography laws, but there are also very good cases to be made for banning possession and viewing of it.
1. If there is a market for child pornography there is a stronger incentive to abuse children. People will produce more of it where it is actually legal to produce (or the legal system is too weak to stop it).
2. There is a strong stigma connected to being presented in pornography. This stigma and the associated injury does not decrease with time. Those who have experienced it describe it as a form of constant, ongoing, abuse that they have to live with their whole life. While you may not mind people jacking off to pictures of children, it is not something the children in the picture can consent to.