"A Little Kiss" adds a massive complexity to its take on the topic with what's become of Megan Calvet (Jessica Paré), now Megan Draper, the girl Don (Jon Hamm) impulsively proposed to at the end of the last season. When it happened, Joan (Christina Hendricks) predicted to Peggy (Elisabeth Moss) that "he'll probably make her a copywriter -- he's not going to want to be married to his secretary" and we see that's become the case. Megan's been put to work with Peggy in the job in which she expressed interest ages ago, though she's mainly there because Don wants her close -- the two are still in a grabby, googly-eyed honeymoon phase in which they roll into the office late and leave early, to the annoyance of those left to pick up the slack.
Megan's become a kind of alterna-Peggy -- the two both started as Don's secretary, but while Peggy's career path stemmed from her talent and Don's mentorship, Megan seems to have pulled off the same thing by being beautiful and hopping into his bed.
It must particularly sting for Peggy because that's how everyone assumes she got where she is. And while she surely knows better at this point than to want any kind of romantic entanglement with her boss, it can't feel great to watch the man who's explained to you he never pursued you because he has rules about that kind of thing in the workplace flaunt how little he actually meant that.
While Joan turned about to be right about Don not wanting to be married to his secretary, that's also exactly why he proposed to Megan after knowing her for only a short while -- in his time of crisis, she was there faithfully anticipating his needs at work and, after his ex-wife fired their nanny shortly before he was due to take the kids to California, at that dreamy, on-the-road equivalent of home. But Megan's a human being, not just a pretty face there to provide support for Don in the office and in his new swank new apartment, and her dissatisfaction with the way things are going comes through during the episode's centerpiece, the surprise party she arranges for his 40th birthday.