Taking a load from Pete internally is dangerous. Legend has it that earlier in his career Pete did a cream pie scene and the girl died because her stomach exploded from his huge load.
This is one of the rare occasions where Pete was having trouble keeping his pecker hard. It might have been his 6th scene of the day and he was tired and overworked or maybe he was so nervous up in that tree that he couldn't focus on his boner. Or Pete was concentrating too hard on his Italian lines that it took away from his sexual performance.