Based on observation I think Google uses a standard much like the one used by the FCC for broadcast television in the United States. Obviously CBS is not broadcasting adult content when they run the Victoria Secret special every year or they would be fined. Just as every other network is not fined for broadcasting Victoria Secret commercials, each of which include models modeling. No network affiliate is fined for broadcasting adult content by showing re-reuns of 30 year old episodes of Charlie's Angels, each of which includes obligatory swimsuit scenes. If Google started objecting to model or swimsuit sites they'd be alone among major media in defining them as adult.