I believe the law does have the right to dictate private behavior such as sexual conduct. I think the example is pretty bad given the problem of rape, molestation, and statutory rape within the United States. Clearly all of these problems have to do with private behavior that involves sex.
The burden of the state to restrict behavior hinges upon the question of who is the victim, and what do they suffer. Any minor is a victim of any sexual conduct, and this should be dictated. Between consenting adults, of sound mind, who is the victim? Just because something pushes what you are comfortable with does not make it wrong, and more importantly, does not make it subject to input from others.
Yes, the laws have a right to dictate private behavior such as sexual conduct, because there are consequences that we all have to deal with. If a young child is raped, they are more likely to die in child birth, because their body isn't ready for it. Children of incest are likely to be disabled. We need society's protections from these consequences.
It should be my right to have sex with any consenting ADULT I choose and not the law. If I want to have safe consensual sex with my sister then that should be my choice and my legal right. I appreciate children and vulnerable adults need the law to protect them which is right.
No, the law should not have any right to dictate private behavior such as sexual conduct as long as all parties are consenting adults. I know there are states with laws on their books which identify certain sexual conduct as illegal. Private behavior is no ones business and should not be used to judge people.