According to Microsoft's published Windows 8 certification requirements, any app rated above PEGI 16 (European standard), ESRB Mature or otherwise contains content to warrant such a rating, will be prohibited from sale on the Windows Store. It's important to note this only affects Windows Store apps and not general Windows software acquired from third parties.
Additionally, metadata -- the publicly viewable information about said apps distributed through Microsoft's app store -- must also follow its own strict set of ratings guidelines. This means titles, descriptions, comments, screenshots and other information published to the Windows Store must not exceed suitability for children under the age of 13: PEGI 12, ESRB Everyone and Windows Store 12+.
Recently, Microsoft and Windows 8 have been well-criticized for what appears to be Redmond's firming grip on third-party developers. Notch, the creator of Minecraft, was the latest prominent figure to express disappointment toward Windows 8 as Microsoft continues to distance itself from maintaining PCs as an open platform. Valve founder Gabe Newell had also voiced his dissatisfaction with the direction Windows is headed.
Although third-party Windows 8 development will remain mostly the same for desktop applications, Microsoft seems to be taking lessons from Apple when it comes to its app store. The Windows Store, much like the App Store or Google Play, will be the only sanctioned means to install software on Windows 8 RT (i.e. least expensive offerings with ARM processors).
Despite Microsoft's firm measures of prohibiting objectionable content, it shouldn't be too hard for users to get their filthy porn fix though -- just fire up Microsoft's own Internet Explorer.