There's no way to stop an underage person getting to hentai sites, either. According to law, as long as you have that disclaimer/warning and keep everything safely censored or segregated, your site holds no responsibility for those who choose to ignore the warning. At least, that's how it was last time I checked. Could be wrong. Either way, you guys know best. Your choice.
I think the biggest problem with implimenting an adult section to Compendium is that it's currently not a site with adult content.
The language on the forums may be so, as well as the humor inside, but the site content outside of the forum is worksafe, and appropriate for most ages.
Changing that format would cause a lot of harm. Parents who allow their kids online may know and approve of this site now, but what will their reaction be when they find a week later it links to pictures of a Nu x Schala gangbang? (Yes there's actually a picture of that)
Some would go as far as legal action over the change of format, no doubt.
I think sticking to the alternate sites (Rule 34, 4Chan, etc.) that already feature adult content is a more responsible route.