You're missing the difference between a moment of weakness, taken advantage of by interested parties, that snowballed into him having to make a choice between two dishonorable deeds and choosing the one that gave him the most personal happiness, and a clear decision -by a grown man who should fully understand the impact of his actions- to renege on his promise just because he met a pretty girl a few weeks ago.
If they chose to portray Robb as increasingly brash and arrogant over the course of his arc, it might make sense. A feeling of "I am their King now, so they'll just have to deal with it." would go a long way to convincing me these are the actions of the same young man we've been watching for two seasons. A young man given too much power before he is ready and led astray by overconfidence.