During season 01 episode 07 of House of Cards, at about 41 minutes into the episode, Lucas drunkenly stumbles into Zoe and in a half-coordinated romantic advance, kisses her.

Frank, watching in his car, makes this face:
[![Frank's face upon seeing Zoe][1]][1]

The background music then changes to a serious tone, so this must have somehow been a significant moment—but my question is, how was it significant? What did it mean?

  [1]: https://i.sstatic.net/QlDrS.png