They led a rebellion to Kings Landing where Ned's sister was killed right? How the fuck could a corpse give birth to a child 9 months later? Unless I am missing something, which is entirely possible, this is the stupidest fucking thing I have read in this thread.
Here's the timeline:
Rhaegar Targaryen, Eldest son of the Mad King Aerys II Targaryen and heir to the throne and all around badass knight dude.
Tywin Lannister wanted his daughter, Cersei Lannister to marry Rhaegar. Rhaegar insulted the Lannisters by declining and said they were beneath him.
Rhaegar instead married Elia Martell, who was Oberyn Martell's big sister.
Robert Baratheon (Bobby B) loved Eddard's sister, Lyanna Stark and she was promised to him by her father.
Rhaegar then hit on Lyanna during a tournament, rustling Bobby B something fierce and the Martells.
Rhaegar later kidnapped Lyanna, causing Bobby B to roll up his squad with Ned and Jon Arryn and start Robert's Rebellion.
During the Battle of the Trident, Bobby B encaved Rhaegar's skull.
http://i.imgur.com/BPEtcrH.jpg
Later, the Lannisters betrayed the Targaryens and let Bobby B's force into the keep.
Jaime Lannister earned the name King Slayer by killing the mad king Aerys II Targaryen.
The Mountain, likely following Tywin's instructions, went to Elia Martell's room and RAPED HER! AND MURDERED HER! AND KILLED HER CHILDREN!! which is why Oberyn wanted to fight him.