What JW said. Whether or not it's cheating is questionable and it is certainly not 'because girl power'. I wasn't happy with the situation but I can't really fault her actions based on what we have been shown of the characters and situations.
Well there was no need for the writer to have the main love interest cheat on the character. Let alone a good reason. Considering what happened to the main character compared to the love interest. On a factor of 1-10 of horrible, terrible things happening to who. He is not a 10 nor a 20 but a 30. At most the love interest is in the negatives. I get Stross wanted to try something new. While going for a less comedic tone. More dark. Yet imo it's at the cost of what I consider character assassination. While angering fans and making a popular character unlikeable. If that character were to die next novel I would shed no tears and actually cheer that the character was dead.