To the OP, much of your argument in terms of a hypothetical child willingly being an object of sex and pornography is the fact of the willingness is what you're ignoring. It's true that such acceptance of reality is hard to do, but the fact is my sister was quite literally a whore at that age. And quite manipulative too (and still is today). Does this mean that one should give up on instilling good sense to one's children (as to support prudence and temperance in all things)? No, it means that one should continue to instill those values that lead to a good life. And not whoring around is a good value to instill, bar none.
Furthermore, the concern that a wouldbe Charlie Manson would get firearms is one that is easily solved without top-down laws: insurance. You sell to Charlie Manson your guns, you get an insurance increase that could be exponential as you are a risk to the other insurance holders. Equally, others that buy from you may get wind of this and not approve, so they stop buying from you. So now you're out of a good steady income and you have insurance that's too high, so you either stop selling to old Charlie boy or you pack it up. There's no way a piece of paper without consequence (aka a law) is going to insure Charlie boy won't get a gun. But putting social and economic pressures on gun sellers *will ensure some measure of caution* in dealing with such men.
So, stop being an ass and oversimplifying the arguments into emotivist gruel.