Because in both scenarios, all the adults are consenting until there is a victim.
"Person C's spouse and children do not have a legal claim on his life that extends to prior restraint."
I'm sure you know, or at least have heard of, families that have been destroyed because one of the parties became obsessed with porn. Rights or no rights, in those cases, the spouse and children are victims.
"Or perhaps we could recognize that correlation does not equal causation"
There's been plenty of evidence, some sited in this article, that porn utilization contributes to behaviors such as rape, child molestation, even serial murder.
There is actually a strong correlation (not necessarily causation) between increased porn and declining sex crimes from 1993-2002. Here are the USDOJ figures:
Sexual Assaults Down by Half Over Last Decade
Overall Decline from 1993 through 2002:
Rape down by 60% (from 1.0 per 1000 persons age 12 and older to 0.4 per 1000)
Attempted rape down by 57.1% (from 0.7 per 1000 to 0.3 per 1000)
Sexual assault down by 62.5% (from 0.8 per 1000 to 0.3 per 1000)
And I'm sure you know, or at least have heard of, families that have been destroyed because one of the parties crossed the street at the wrong time and was run over by a truck. I suppose the spouse and children are victims of the offender's reckless and risky decision to cross the street. Do families have the right to use the government to enjoin their members from engaging in risky behavior such as smoking, drinking, or crossing the street?
Of course you know my answer. Please note that this does not preclude them from issuing personal injunctions against such behavior, in the form of, "If you don't stop [drinking|smoking|yanking it to porn|eating those fatty foods|crossing the street], I'm taking the kids and we're outta here". But the minute they start asking the government to get involved, I have a problem with it. Government is force, every government mandate or injunction has an implicit gun behind it. And it's immoral to prevent someone from engaging in a victimless crime because he might offend his loved ones by pointing a gun at him.
There's been plenty of evidence, some sited in this article, that porn utilization contributes to behaviors such as rape, child molestation, even serial murder.
There's plenty of evidence of correlation between consumption of pornography and certain antisocial behaviors. The evidence of causation is far shakier, and is often from sources of questionable neutrality.
Maybe consuming pornography makes you more likely to be a serial killer. Or maybe being a serial killer makes you more likely to consume pornography. Or maybe there are a certain set of mental traits that make one more likely to both consume pornography and commit serial murder. There's really no way to know for sure; the data support all three conclusions. And only the first one would even remotely come close to justifying restrictions on pornography in the name of public safety. In fact, in the other two cases restrictions on pornography might actually be counterproductive and decrease public safety.