What’s interesting is that Megan’s use of the qualifier - you know - in her declarations of love for Don - either as a way of convincing herself or him - hasn’t changed since “Tomorrowland” as she continues to use it in “The Phantom.”
Season 4 finale, “Tomorrowland”: “I love you, you know.
Season 5 finale, “The Phantom”: ”You know, I love you.”
Ergo, her feelings of love for Don have neither developed nor deepened after their year together which means she doesn’t love him now and most likely never will; at this point, she remains married to Don to use him in order to advance her dreams as an actress.
In Don’s case, although he’s remained faithful in his marriage thus far, he also likes the beginnings of things and as the resident sage Joan Holloway tells him, he “is who he is” and thus his infidelity clock may have subconsciously started ticking the moment he said, “I do.”