I would say that Brandon/Ashara was the norm, but I’ve always had a problem with that particular theory. To me, there’s two key limitations to Barristan as a witness: first, he’s a giant romantic so he’d much rather think that a Stark “dishonored” his beloved than she willingly slept with someone else and therefore wasn’t the pure embodiment of the Maiden he had in his head; and second, he wasn’t particularly close to the situation – Barristan was busy fighting in the tourney when he wasn’t guarding the king.
So what I think happened is that Barristan heard a story about a Stark sleeping with Ashara and jumped to the same conclusions that a lot of people have, on the basis of rather superficial judgements about the people involved.