I just wish I could agree with you about Frank and Claire's relationship, but let's review:

Without citing specific examples, absolutely every single person in either of their lives is completely disposable to them. Even anti-social couples who love each other tend to have a friend or two they confide in and would consider irreplaceable, yet there is no evidence either of them has any other such contact, even within their own families (even though most of Franks family is dead apparently).

plus, both are.. and I'm probably using the term incorrectly as I've read multiple definitions, text book narcissists and sociopaths, albeit highly functioning sociopaths. It's not that they don't love each other, it's that they simply don't have the capacity TO love as neither of them is capable of self-sacrifice. They simply don't form emotional connections with people but UNDERSTAND that other people have these connections and how to effectively use these connections against them.

They UNDERSTAND that people love other people and both probably view this as a weakness as they consistently use this against their peers and opponents, either in the form of manipulating them to do something for a loved one or suggesting they will create an image of their victim as a sociopath, or someone who simply doesn't care about people, at all, ever. This is very unsettling for the victims because the sudden reversal of the Underwoods appreciation of them is so dramatic.

That and most people don't ever really encounter or have long term relations with Sociopaths, so most people don't believe that people really exist who are really like that. That's what movie villains do and isn't something that happens in the real world. In a sense it's like an upper class white person in America saying they relate to the kind of outright racial hatred a black person in the south during the 60's may have felt, I mean sure, maybe one time that guy seemed to hate you for no reason, but a sea of strangers every single day openly despising you for who you are is simply unbelievable, EVEN TO THE PEOPLE WHO ARE ACTIVELY HATING THAT PERSON.

Not ONCE have I heard them speak to each other as lovers, always as a very polite, very formal business partners. Even their displays of emotion in private seem contrived.

And as a disclaimer here... I'm not saying sociopaths are all bad people, some function not unlike a healthy adjusted fully emotionally aware human being.