Season 3, Episode 5: Kissed by Fire
Jon and Ygritte have steamy cave sex, thus breaking his oath.
After what seemed an eternity of sexual frustration, Ygritte – the cute, fierce Wildling redhead who he’d previously taken captive – is able to lure Jon into a cave to seduce him. Up until now, Jon has been attempting to gather information from the Wildlings under the guise of having defected from The Night’s Watch. Jon isn’t a deceptive fellow by nature, but this particular lie was in the service of a greater good (protecting The North from a Wildling attack). Someone with less knowledge of Jon’s immutable honor could see his rendezvous with Ygritte in the Love Cave as a way to keep up the ruse, but we all know Jon better than that. This is a man who took his oath so seriously that it kept him from taking up arms with Robb to avenge their father. So, when he breaks it with Ygritte, we know that this must be the real deal; if it wasn’t he could have brushed her off the way he did – repeatedly – when she was his captive.
What we see here is a man who will ultimately be led by his heart. There’s a great moment with Aemon Targaryen in Season 1 where he asks Jon whether he would follow his family or his oath to The Watch when forced with the decision. Jon responds that he would do “what was right”….So, having steamy cave sex with Ygritte must have felt pretty right.