A webmaster asked in a Google Webmaster Help thread how would he be able to build an adult site that has a mix of content between adult and family friendly content on the same site.
The main thing to think about here is how to make it easy for Google and for users to determine structurally where the family friendly content is and where the adult content is.
Googler, JohnMu, suggested he go with unique domains or subdomains. If you can't go that route, then try subfolders. Here is John's advice:
Generally speaking, if you can, I'd recommend separating your adult content from the more general content, so that it's easier for search engines to separate that on their side as well. Using a separate domain or subdomain is a pretty clear sign, subdirectories alone may be little bit harder to recognize (but if that works for you, why not), and using URL parameters is probably even harder. One thing I would watch out for, especially with a site focused on user-generated-content, is that you keep your homepage clearly adult or non-adult focused. For example, if you were to list posts & photos from all parts of your site on your homepage, it might be that we end up classifying your homepage as being adult-oriented, which might not be what you'd want.
As you can see, Google would prefer you use a completely separate domain and if not, then go with a subdomain. If that doesn't work, then go with subdirectories and if you can't do that, then try special URL parameters.
Also note John's comment on the home page. If you do not want your home page to be associated as adult, then you need to think this out a bit.
Forum discussion at Google Webmaster Help.