You know, honestly - I like the idea of it. The sticking point is that it creates complications which we've yet to find a suitable workaround for. We do have some services in the marketplace that offer adult related services, although they're restricted to using appropriate or censored imagery and content to describe it.
Even without an adult section, the forum intermittently gets banned on certain ISP's or wifi networks as having adult content or being about "illegal" topics. When we're notified, we'll try to contact the relevant people and get them to review the classification. Being classified as "adult content" means that people in places with any level of net censorship, or who use ISP's with censorship agreements - could have limitations on their access to the forum.
Not only that, we have a lot of junior members on the site coming from all over the world, where legal age limits for adult content greatly differ. Some countries and US states set the minimum age for adult content as high as 21. I've had quite a few outraged support tickets from adults who've found out their darling child (or "totally innocent" 13 year old student) has been accessing the forum or received an "inappropriate" email notification. Some of our most active, knowledgeable or helpful members are under 18. Setting an arbitrarily high age limit to match the adult content seems too restrictive.
Lastly, there's potential issues of being penalised by the major search engines for having adult or inappropriate content.
Ultimately, we don't have any objections - in principle - to adult content or an adult section but we haven't yet found a good way to provide it fairly without negatively impacting existing members. One option could be that it's placed within one of the private sections of the forum like the Jr VIP section. However, it seems restrictive to only make the discussions available to paid members and it still doesn't address the problems it could cause for members under 21.
To summarise: it's not in place because we don't think it's a good idea - we just haven't found a good way to do it while avoiding the above issues. Having said that, we're all ears if anyone's got suggestions or ideas to deal with them.