Sorry to insist, but there’s no evidence in the books for premarital sex being accepted in Westeros; in fact, we can read the opposite and the importance of the “maidenhood” (Tyrells had to say Margaery never did Renly). I insist, it’s probably the main reason why Robb ditched his vow to marry the Frey girl. While you may argue that Ned could have been more impulsive and less honorable as a young man, I don’t think you have a strong case for premarital sex being ok for higborn women of Westeros.

Harwin’s speech in ASOS is evidence precisely of that:

“When Ned met this Dornish lady, his brother Brandon was still alive, and it was him betrothed to Lady Catelyn, so there’s no stain on your father’s honor. There’s nought like a tourney to make the blood run hot, so maybe some words were whispered in a tent of a night, who can say? Words or kisses, maybe more, but where’s the harm in that? Spring had come, or so they thought, and neither one of them was pledged.” (ASOS)

I’m not saying it’s considered ok in all circumstances, I’m saying there’s a loophole in one specific circumstance* – when the two get married afterwards. Robb marrying Jeyne doesn’t make sense if the two of them having premarital sex isn’t cancelled out by the marriage, for example. While we’re talking about Margaery, it’s pretty clear that the Reach has an “anything but” policy when it comes to sex before marriage – and even that didn’t stop Olenna Redwyne from snagging Luthor Tyrell. 

And in this specific circumstance, the dishonor for the woman in question is mediated by her culture. “There are other customs besides that mark the Dornish as different. They are not greatly concerned if a child is born in wedlock or out of it, especially if the child is born to a paramour.” If Ashara Dayne wouldn’t consider an affair dishonorable, and a Northman like Harwin wouldn’t either, why would Ned?

* and even then, the dishonor is not immutable. Gatehouse Ami is still able to get a good marriage with lands, Walder’s sister ditto.