Read original post by Tracey Lien at The Verge
The Windows 8 Marketplace will not be able to sell any apps or content with an adulting rating, including video games that are rated 18+, according to the Windows Store guidelines.
Under section 6.2 of the Windows 8 app certification requirements, apps with a rating over PEGI 16, ESRB Mature, or a corresponding rating under other ratings systems as described by the table in the guidelines are not allowed.