I felt very sympathetic toward Frank even with his multiple affairs and his sometimes shortcomings in his dealings with Claire. He was very much in love with Claire, and we really weren't exposed to the great loss he must have felt when she went missing. I can't imagine the hell he would have lived over those few years when she literally dropped off the face of the earth (at least his earth). Then, with Claire returning pregnant and in love with another man, even if that man seemed a figment of Claire's imagination, he still tried his best to stand by her.
He loved Brianna with his whole heart, and in the end, tried to support Claire in her quest to become a doctor. This, before it was quite common for wives to work outside the home, especially in such a demanding field. Obviously, Claire couldn't give Frank her whole heart after loving Jamie with such intensity, so I was sad for Frank that he didn't find another woman to love him and he her, rather than settling for a string of extra-marital affairs. I rarely sympathize with a cheating spouse, but this was such a complex set of circumstances, that I found myself very sad for the life Frank ended up living. On the other hand, had Claire not had the relationship with Jamie, Frank would have never had the wonderful gift of being a father to Brianna.
We never got to know Frank that well, but I imagine he would have remained faithful to and in love with Claire had she not fallen in love with Jamie. Of course this is only a novel and we can build the little world outside of Gabaldon's vision of her characters as we wish, but I believe that had Claire actually fought for Frank's love and given hers to him in return, the affairs would have ended and Frank would have been a loving husband as he was a father. Then again, this would have been a whole different and much shorter story, and I wouldn't have gotten sucked into reading these books hours on end...lol