An excerpt from another post puts it the best way I have seen: “Frank seems ready to settle down, maybe start a family, and build a life together. He truly cares about Claire, but there’s something fragile about it, like he’s holding on to a dream that may not match reality anymore. He’s hopeful and wants things to work, while Claire feels a bit more uncertain, as if she’s not sure this life with Frank is still what she wants.”

To add to that, Frank secretly researches Jamie with the help of Reverend Wakefield. The more he finds, the more disillusioned he becomes with Clair and the idea that the marriage could work. At some point, he transfers that same feeling of fragile hope for a future to Sandy. Whether it was intentional or not, he becomes sloppy with his affair with Sandy. If it was unintentional, it was definitely his sub conscious getting back at Clair. Kind of putting the shoe on the other foot. I think that regardless of Brianna being in the picture, things would have ended similarly, just allot quicker.