The Bethesda Softworks Facebook page has word that the ESRB has advised them that The Elder Scrolls Online is getting an "M" rating, designating it for mature audiences only. They say they disagree with the rating, but do not plan on challenging it because they "are unwilling to change the game’s content to achieve a different rating." They note they are in the process of updating the game's marketing materials to reflect this rating. In the meantime, there's this video where three of the game's developers discuss group content in a clip which runs almost eight minutes. The clip does not end with an age-rating, perhaps the last time the game will go commando. Similarly, the ESRB listing for the Thief reboot also shows that game with an "M" rating (thanks Lazygamer). The description suggests violence plays a part in the rating, though, similar to many rock stars, it was the sex and drugs that did the game in:

During the course of the game, players can roam through a brothel environment in which several prostitutes are depicted topless. The central character has the ability to spy on couples engaging in sexual activity (topless characters); suggestive moaning and dialogue can also be heard. Some settings depict opium vats or characters under the influence of the drug. The words “f**k” and “sh*t” can be heard in the dialogue.