SANTA CATARINA, Mexico — Sex never has been safer since a motel called the Thunderclap Ranch reinforced its cinder-block rooms and started advertising them as bulletproof. For $5 an hour, guests here in the gritty outskirts of violence-plagued Monterrey needn't worry about jealous spouses — or more to the point, drug cartel hit men, who usually are better armed. Employee Lucy Regalado, 50, stands inside a room at the 'armored' Thunderclap Ranch hotel in the violent Monterrey area. Business is booming at the $5-per-hour establishment, which also will supply six-packs of beer for $4 and snacks through a porthole in...