Stannis never receives Ned's letter about Joffrey's parentage. He's not the type to listen to scurrilous gossip (or the rumour never really spreads without him). He remains loyal to Joffrey, the rightful king... even as he realises the boy's a monster.

+ Renly still makes his bid for the throne, feelings are had (Stannis doesn't really want to fight his little brother for his insane nephew, but it's the right thing to do).
+ His reaction to the Red Wedding (he thinks Robb Stark is a traitor, but for his allies to have done something that dishonourable...)

http://asoiafkinkmeme.livejournal.com/22515.html?thread=16602867#t16602867