Catherine Zeta Jones!
Shame on Michael Douglas for cheating on her! How do you cheat on Catherine Zeta Jones!?
Same reason someone would cheat on Elizabeth Hurley, Halle Berry, Eva Longoria, Sienna Miller, Reese Witherspoon, Jennifer Aniston, Katy Perry, Sophia Bush, Jennifer Garner, Demi Moore, Britney Spears, Christie Brinkley Sandra Bullock, Jennifer Lopez, and others.
Things are different when you are famous and have opportunity. Like if I had a chance to date my top crush Hailee Steinfeld, I'd never consider cheating on her. But if I were another celeb who was dating her and had a bunch of fan girls throwing themselves at me and/or other hot celeb, who knows?