About Ashara and Ned: there was dishonor, because sex before marriage is dishonorable. And Catelyn was “in love” with Brandon, but as far as the evidences show, Brandon didn’t care about her. Apart from the duel with LF (which is not about feelings, really, more about honor), nothing happened showing Brandon’s affection. Dustin says he didn’t want Cat at all, and seeing the womanizer he was, I believe Brandon wasn’t interested in Catelyn.

Sex before marriage isn’t dishonorable. Harwin says as much: 

“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.”

As for Brandon, while I’m not sure whether or not he “loved” Catelyn, I’m pretty sure he was attracted to her. Catelyn was a beautiful woman, everyone pretty much agrees on that, and womanizers normally are attracted to beautiful women.