What you're missing is that there are many people who think these things are illegal because "ew that's gross" and they also elect politicians who agree with them.
The United States has barely moved past the issue of homosexuality even though it harms no one, it is perhaps the most straightforward modern example of people trying to ban it due to personal & traditional disgust. Although there is a general majority approval for it, that doesn't carry to the electoral level where it remains a contentious partisan issue. But you think the ethics of CP & pedophilia will be discussed and legislated rationally?