Like the man in the famous Milk Tray ads, it was "all because" he was trying to reach a woman he had the hots for. But unlike the telly commercials, Robbie was in his birthday suit, blind drunk and incredibly horny. The outrageous stunt has been revealed for the first time in a blistering new biography on the megastar. COCOA-NUTS: The hotel he dangled from PHOTO: JTB Photo …