Well, you just cheated on her, she chose to forgive and stick with your character(because she is fucking devoted to you at this point and has to deal with it). Doesnt really matter if thats "how she was raised" statment is true or she chose to use it as coping mechanism. You can chose meanest options in other dialogues but she usually just goes "I will manage, we dont break up".
No, Karlach chooses to forgive depending on dialogue options, Shadowheart didn't 'choose to forgive', she's fine with it and just wants you to ask next time. And I just edited in a bit late to the previous post because I forgot to mention it, but it makes no sense that she doesn't expect you to remain faithful considering she just spited her only other support in life, which was a literal goddess, just to devote herself fully to you. She has enough faith in you to think you'll lead her to a better life than a goddess could, but not enough faith in you to think you'll not cheat on her?
She is insecure whole act 3 though. Still the way she places emphasis on "swan" line makes me believe that she is not fine. And after this she just accepts that she has to deal with it and only asks you to ask her first if you plan to do anything like that in the future. Also her acceptance is played perfectly in the last line when she responds to "And you would've said yes?" - "You gotta eat. What do I get from denying your appetites?"(She doesnt say yes. She is also sad when she says that btw).
