She played the part of housewife, forfeiting her own career for his and being faithful, until he got caught in a scandal where a hooker both charged him and also took money from political opponents who were out to bring him down. She stood by him during the time he was in prison. She decided to stick with him as she thought he was a reformed man and also since the hooker was being paid to come onto him, she viewed him as something of a victim of a plot more than a philanderer. As her own career took off, she grew close to her old boyfriend Will from law school but held off consummating. Then when Peter returned to his old tricks, she started the affair with Will.
Basically, the husband started the messing around and the power games, so at some point she felt she had the right to have her own life. The kids were still in school so divorce, (which she was going to do after Peter got out of jail), was put aside. You're right, it is complicated, but both were put in the political spotlight, they had to worry not only about themselves but about how things affected their political career.
They aren't perfect, but I don't see them selling out any great priciples they believe in unless the law compels it. What I was put off by was the legal and political professions and how yesterday's friend and buddy becomes today's enemy who has to be taken down, regardless of how viciously it has to be done.