Earlier this month online media noticed that Microsoft's Windows Store guidelines did not allow any PEGI 18 rated games, even though many of these games are rated ESRB Mature in the US, causing a major discrepancy between the content available to US and European customers. For instance, Skyrim, Call of Duty, Borderlands 2, and Max Payne 3 are all rated PEGI 18 in Europe and would thus be banned from the Windows Store.
Microsoft has taken notice and has revised the Windows Store policy to allow games rate PEGI 18:
Now, Microsoft has revised their stance on "mature" content in the Windows Store, and they will in fact allow games rated PEGI 18 onto the Store to bring content available in Europe on par with that in the States. Antoine Leblond, Windows Corporate VP of Web Services, told Gizmodo that the change will take some time to come into effect - the content won't be available straight away, but hopefully by December.
Lebond hopes that by informing developers of the changes to the Windows Store policy, they will "have the peace of mind" to continue developing games with potentially mature content.
Adult content (aka porn) remains blocked in the Windows Store.