I'm not a psychologist and I don't play one on TV. But as a writer, I'm endlessly interested in why we do the things we do because that's the stuff of character.
A great plot is essential to a story because it moves the story forward, adding suspense, mystery and intrigue. While a good plot will keep a reader interested, it won't make the reader CARE as much as great characters will.
Readers will invest in those characters whether they sympathize and identify with them or whether they fear and loathe them. Either way, the reader cares what happens and wants to understand why the characters are the way they are.
All of which is preface to trying to make sense of the Arnold Schwarzenegger story. My take is based on a terrific book, Mistakes Were Made But Not By Me, written by two real psychologists. They focus on cognitive dissonance, the uncomfortable feeling of simultaneously holding conflicting ideas or beliefs.
In a case like this, Arnold may have simultaneously believed that he loved his wife and that he wanted to have sex with this other woman. To resolve his cognitive dissonance, he may have utilized self justification to rationalize his behavior, e.g., "I'm entitled because of who I am," "My wife doesn't understand me," etc.
Of course, there is another possible explanation. The guy is a total asshole. Some people just are and it's no more complicated than that. But who am I to say for sure? This psychology stuff can get too complicated even for a writer.