The love triangle aspect of Outlander is a complicated and touchy subject. In the fandom there are FoF (Friends of Frank) and Team Jamie and people are very passionate about their allegiance.
I think the reasons for the this loyalty has to do with the complicated moral issues of the book itself. Diana Gabaldon created this love triangle in which Claire is married when she falls through the stones into a new time period and must marry again. She falls in love with new husband, Jamie, and chooses to stay with him over Frank.
This creates a difficult situation for the reader who must now sympathize with a woman who may or may not be committing adultery. This was a challenging moral dilemma for me when I first read the books, and almost caused me to stop reading them. While it may be easy for some to justify Claire’s actions: she was forced to marry Jamie, or Frank was not alive in this period, another justification that many people cling to is Frank’s infidelity (or just plain: Frank was scum, to put it nicely).
Diana Gabaldon plants a seed of Frank’s possible unfaithfulness in the mind of the reader early in the book. When Jamie’s ghost appears and is watching Claire in the window, Frank just sees a man who is possibly in love with his wife and asks Claire about it. Claire then supposes that maybe Frank jumped to this conclusion because he was the unfaithful one. This is Claire’s thought, and the reader has nothing to prove or disprove it.
I think most readers have a need to accept Frank’s unfaithfulness here (and in a later book) in order to make Claire’s own unfaithfulness okay. Because if Claire is the 2nd person in the marriage to be unfaithful, than she is not the actual guilty party.
I also know that we never read a book in a vacuum. Our own experiences color our interpretation. And because half of married Americans divorce, and adultery is a main reason for divorce it makes sense that a lot of readers may see in Frank their own unfaithful ex-spouse. They may feel a strong conviction because they saw the signs of infidelity in their own marriage. This is really the only explanation I can come up with that explains the extreme hatred that I see and hear toward Frank online and in conversations.
I was firmly a friend of Frank on my first read through because I saw Claire as a married woman in love with her husband and desperate to get back to him. I saw her marriage to Jamie and falling in love with him as a betrayal, even though I really liked Jamie and thought he was an innocent and wonderful man. As I continued reading the series my own views of Frank followed right along with Claire’s view and when she suspected he was unfaithful, so did I.
Then one day I read in an online forum that Diana Gabaldon said Frank is an honorable man. An honorable man would not cheat on his wife, in my opinion, so for her to say that might mean that Frank was not a cheater. I decided that maybe I should do a read through of the series looking at Frank as a faithful husband, instead of as a cheater.
Now, I know about the “death of the author” and that no two people can read the same book because we all bring our own stuff to the book. But I also know that I don’t have to keep my preconceived notions when I read a book. I can toss those out, consider the fact that Claire is not a reliable narrator, and read the books looking for evidence that he is an honorable, faithful husband.
There is a lot we just do not know about Frank, and I for one, am hoping someday Diana will tell his story.
For now, I can develop a my own theory of how it is okay for Claire to have two husbands and love two men. For me, that meant looking to the words of Father Anselm in Outlander, but also thinking about the stones as an uncertain means of travel. After Claire’s first journey, she really did not know for sure that she would go back to Frank if she went through them again. That had to be enough of a reason for me to justify her staying with Jamie. And with Frank not in the 18th century, it would be the same as if Frank were dead, and Claire a widow.