Bowen,
I think there is some merit to your arguments, but where do you draw the line between offensive and not offensive, and how do you ensure that the standards are enforced uniformly?
Forums in general tend to devolve into a Lord of the Flies situation, augmented by the relative anonymity, and the security of not having to look someone in the eye when you grief them. I would say that while Pinside does occasionally venture into the area of the unsavory, for the most part, Pinsiders tend to be a heck of a lot more civil than the rest of the internet.
The problem I see with moving "adult" themes to a private area is that it would likely only apply to initial threads and posts, which doesn't really help prevent the more egregious stuff you're talking about.
And, if you do flag topics due to individual replies, you run the risk of burying a lot of good discussions behind a barrier, and you give the mischievous Pinsiders incredible power to sink topics just by posting something others might find offensive somewhere in the thread.
And then there's the issue of what people find offensive, and what constitutes vulgarity. While this is a privately-hosted forum, and the admins can set whatever limits on speech they want, the problem is that in several first-amendment cases, no one has ever been able to precisely define what obscenity is, and at which point is crosses the line. If we start weeding out posts that someone finds unsavory, pretty soon, everything is getting the banhammer.
I do have a few ideas, though, that may address your concerns.
1) Eliminate user names and require everyone to use a verified first and last name. I can't see that going over well with users, but people tend to be a lot more civil when they know their employer can find their posts in a Google search.
2) Set a rigid set of guidelines that the mods can spot-check threads to enforce: specific sanctions for posts of a racist, classist, sexist, homophobic, xenophobic nature, or the posting of images that are of an explicitly sexual nature, or which portray violence.
Beyond that, though, I think that most parents know that they need to curate their kids' browsing.