I know she cheated on Peter and she definitely sucks for doing that, but man did she just get destroyed by the end of that movie. It really seems like she tried really, really, really hard to make it work with Peter by going to different therapists and stuff. Peter was stuck in a rut and was dragging her down with him. He didn't seem like a good boyfriend at all.
I just thought it was so sad how he only turned into the guy she needed him to be after they broke up. All her efforts in helping him become a better guy ended up making him better for another girl. At the end of the movie, she's unemployed, caught an STD, got cheated on and broken up horribly by a shitty boyfriend, and is forced to watch Peter become the guy she's worked so hard to get, only for him to end up with a different girl.
I feel like they only added the whole cheating thing to make her the villain. Had they not included it, it would in my opinion have made Peter the villain. A big reason why I feel this way is because of Kristen Bell's acting in that one scene where Peter says "If only you tried harder, we could've worked out." and she goes "I tried... Believe me I tried. Don't you dare say I didn't try." just made me feel so bad for Sarah Marshall, despite my very strong hatred for cheaters.
What do you guys think?
[Edit] To be clear, I don't think what she went through justifies her cheating. I just think the cheating was necessary for the story to work the way it did. Otherwise it just makes Sarah Marshall a girlfriend who got royally screwed for wanting a better life. This is interesting to me because they didn't reveal her cheating until near the end. Imagine how much different the movie would be if she never cheated on him.