Heracles (Hercules): Birth and Early Adventures
The Birth of King Arthur
Zeus isn't the only guy from mythology to seduce a woman while disguised as her husband. It seems like his seduction of Heracles' mother in the guise of her husband may just have inspired some similarly seedy shenanigans down through the years. For example, King Uther Pendragon, father of King Arthur, pulled the same trick on Igraine, the wife of his enemy, Gorlois.
With the help of Merlin, the famous wizard, Uther snuck into the castle of Tintagel magically disguised as Gorlois. Igraine unknowingly conceived a child with her husband's archenemy. It's interesting that both King Arthur and Heracles, two good guys, were conceived with what most would probably consider pretty bad methods.