The major point on the network-wide policy on these materials is the following line from the Content Policy:
Sexually Explicit Material. Accounts that use Stack Exchange to post sexually explicit or pornographic material, or links to it, will be suspended.
That’s really the root of it, network-wide. As noted in ChrisF’s answer, this kind of action could lead to us being globally blocked or otherwise marked on the entire network. No posting explicit material, no linking to places that host explicit material, no directing people there, and no trying to be cute with words to try and hide the fact that you’re still quite literally discussing explicit content.
There’s a good portion of adult content that can be discussed without crossing the line of “sexually explicit”, however. We know it’s a thing, we’re not going to pretend that it doesn’t exist. And the fact remains that some sites actually need some degree of ability to talk about this kind of stuff - and not because haha let’s talk about adult stuff but because someone has a genuine situation they're trying to solve for within the scope of that site, and there are a good number of other people who will benefit from the information being available.
That’s why, past the notes on the Content Policy, we mostly let sites be the judge of what they want to allow on their own site.
Arqade for example very early on mostly decided to not permit strictly adult games - allowing just the non-explicit content to be asked about puts the site in a very weird position where we’re quite discoverable for people looking for a game but can’t answer 80% of the possible questions where that 80% is why some portion of the people who play those games do so. Adult content found in other sorts of games is then mostly handled on a case-by-case basis, mostly sticking to the network content policy.
Movies & TV, meanwhile, permits adult movies as a topic of discussion. They allow it simply within the confines of the network content policy, so one can’t talk about the explicit portions or link to such things but anything else in the movie is fair game. To date, only 4 such questions exist - 3 of which are general informational questions and 1 being the only surviving identification question (the vast majority were automatically cleaned out by the system due to inattention).
Other sites, like Parenting or English Language & Usage, abide by similar principle because even within their own domain, there’s simply going to be times when you need to talk about this stuff. Both of these sites actively dissuade against slang or regional terms, preferring to stick to clinical terminology (unless the question itself has to do with slang/etc).
So as a general rule that I’ve pretty much rephrased in every paragraph before this one and am doing so again here, we have a network content policy. Stay in those lines and you’re good. No explicit content, neither directly on the sites nor directed to from the sites. After that, it’s up to the community to decide how they wish to handle the stuff, if they want to have additional rules or not.