Did Eddard Stark really cheat on his wife, or was he protecting his sisters son? Who is Eddard Stark? Why did he not tell John Snow who his mother was? Why wouldn't he tell Robert Baratheon who the mother was...was it because Robert Baratheon bashed John Snow's father's head in at the Trident? And that Robert was willing to assassinate Daenerys when she was really just a girl? And why did Eddard object so strongly? Why was he offended by the idea of killing all of the Targaryen heirs?
And just think back, Lyanna Stark was supposedly kidnapped by the Targaryen's, but in fact she ran off and eloped with Rhaegar, got knocked up and had a baby. Eddard found the baby and vowed to protect it at all costs. He lied to his wife, claiming he had a *******, all to protect his sister's child. There is no reason other than that which could motivate him to lie to his own wife. And we all know, Eddard isn't a cheater. Not in any way shape or form. He was loyal above all else. Only to protect his sister's child would he allow everyone to be deceived into thinking he was other than he really was.