I would agree with you that it probably wasn't her first thought. Her first thought probably was along the lines of, "I'm going to castrate you, you lying sack of you-know-what." The staying with him for political gain was probably her second thought.
I jest of course...
But when you're faced with making a decision, you either consciously or subconsciously weigh the pros and cons, figure what the payoff is. Honestly, I can't see much payoff staying with a man who has serially cheated on me and embarrassed me to the nth degree. You're right, we don't know what goes on behind closed doors. I never have liked Hillary, so honestly, I skew my opinion of her motives unfavorably.