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.
I'm not. I'd detailed above how I felt the latter made more sense for an older Robb, the argument at least assumes I consider them substantively different. But a Robb breaking his oath because he'd had a ravished a maiden is not inherently more honourable or, in the feudal sense, less deserving of the Frey's wrath than a Robb breaking his oath because he met a girl.
To phrase this another way, taking the virginity of a noble maiden is in itself
also a dishonourable act. Robb is not caught between two abstract dishonourable choices in the books, he makes a rash, impulsive, and hormonally addled choice of action that is dishonourable and leaves him with different recourses, either of which compounds the existing dishonour.
Now, that something is dishonourable is not the same thing as saying it isn't something we might excuse as the impulsiveness or the weakness of a teenage boy... but that doesn't stop it from being what I said above.
Breaking a betrothal isn't quite as severe as breaking a treaty, and isn't that big a deal. No one complained when Joffrey broke his betrothal to Sansa, after all.
Joffrey's breaking of his oath to Sansa works because they covered their asses thoroughly. The justification that Sansa's family had betrayed the crown is given as just cause (the Freys are not in open rebellion against the Starks, on the other hand) and on top
of this Joffrey gets the special dispensation of the High Septon to go back on his vows.
It's a feudal society. Marriage is politics is marriage.