WARNING: THIS ARTICLE CONTAINS SPOILERS.
In the episode in question, Vice President Frank Underwood returns home to find his wife Clare and her bodyguard Meechum drinking together. It is clear that Frank has walked in on an attempted seduction, something that isn't out of the ordinary for their sexually liberal relationship, but instead of walking away and leaving them to it, Frank lets it play out in front of him. And just as Clare makes her move on Meechum, Frank gets involved too, before the camera pans to way to leave them to what is presumably their ménage à trois. The whole scene can be watched here:
Because House Of Cards is made by the On Demand streaming platform Netflix, there are no constraints set by the networks as to what they can and cannot show. This freedom allows them to explore topics and themes that live television broadcasters would feel uncomfortable with and this is definitely symptomatic of that. It's not to say that live broadcasters don't want to show LGBT characters and issues - for from it in fact - but Frank's behaviour is hard to pigeon-hole. This isn't safe and family-friendly homosexuality - this is a character in a heteronormative relationship not just acting outside of his marital bonds, but doing that with someone of the same sex... with his wife's permission... while she watches. And this isn't some supporting player in the TV series; this is your protagonist.
Some people might argue that because Frank is clearly an antihero, it's his deviousness and immorality that make us sadistically root for him, regardless of what he does... but we have never seen an antihero's questionable morals extend sexually to cross genders. And, to audiences, this is making little or no difference. We don't judge Frank any more or less for his being bisexual, nor do we judge Clare for allowing her husband a tryst with a man now and then; it's almost irrelevant and it's this irrelevance that is incredibly relevant to any LGBT audience watching. No one is going to turn off House Of Cards because its protagonist suddenly slept with a man. It's the realisation of this that is what makes this Netflix Original all the more seminal, all the more important and all the better for it. Just as sexuality itself is fluid, Frank is a malleable chameleon and adding this sensational side to his personality makes the show all the better to watch. And after years of LGBT characters becoming homogenised and watered down and caricaturing themselves, in response to Clare, we needed that too.