I answered "Yes - Other reasons". Surprising, is it 
I would have liked to see her leave with Robert. Her husband was boring, and yes, Robert was hunky, but that has nothing to do with it. Robert and Francesca's connection was not only physical. It was a meeting of the souls. They were soul mates.
It was a once-in-a-lifetime-chance to be happy, at last -she was not -has she ever been, really? ... and when you have this opportunity, you don't want to let it pass you by, even if that means making life shattering decisions. Francesca made a choice, and I respect that.
But it was fate. They were made to be together.
By the way, what's with the question "Should Francesca have left her faithful husband for Robert?" Faithful? Were you trying to influence us or something?
It doesn't say in the movie that her husband was faithful -or unfaithful for that matter ... right? That's just what you think
I'm not saying he was not faithful, but you could have left that out when you wrote your question -for the sake of impartiality.
There's still one thing that I can't understand -here I go again. Why, when we talk about this film, you guys only think about cheating cheating cheating. I mean the movie's deeper than that. It's not the story of a woman who's cheating on her husband ... is it? 
BOMC is a love story.
Why can't you see beyond the cheating aspect of the film?
I agree AB. My thoughts on it are pretty close to this.
I voted Yes...this was true love. I think that cheating is wrong, and this is the only case (fictional or otherwise) I can think of where I felt it wasn't wrong. They were soul mates. Francesca was unhappy. She wasn't loved the way she needed or deserved to be. Robert loved her that way. Being with him, if for only four days, was her chance at happiness. I read Waller's A Thousand Country Roads, and in it there was a line that kinda summed that up. "One great love in a lifetime was enough for anyone." Even though their time together was brief, their love lasted forever, as they still loved each other til death. It's what sustained her on the farm all those years, too.
I wish she would have left with Robert, but I know that if she had, the story wouldn't have had the impact that it did. Sad endings are usually stronger. I don't think she needed to tell Richard about what happened, either. If she would have left with Robert, then yes, I think she should've explained to him why she was leaving. But that's the road less traveled..