The whole episode dealt with the power, or lack of it, some people have over another. Therefore, it was the perfect episode to tie in MLK's "I have a dream speech" as the lack of power and how this time was just begining to change for blacks tied into the characters' lives.
Don was getting battered at work as Hilton was calling at all hours and it turned out has almost impossible demands. For one of the first times, Don was at someone beck and call, and he revolts by seducing the teacher, who he knew he could assert power over. Don's relationship to Hilton is alpha beta dog, almost father and son like. Hilton states after he dissaproves of the campaign Don thought out "if I want the moon, give me the moon". Its an impossible request, and Don sees he is subserviant to Hilton and badly needed to reassert his Alpha-dom, which lead to the probably distasterous affair with the teacher.
Betty also sees, almost too late, she is also at Henry Francis' beck and call, crashing into his office wondering why he didn't show. He unknowingly ruined his chance by speaking plainly and told her he couldn't because she is married and she had to come to him. She realizes he is correct, which is why she permitted one kiss, but then realized the romantic notions she had cannot be brought to fruition because its impossible. She wanted an 1800's romance with love letters, but she is married with three kids. The only way for her to cheat is for it to be "tawdry". Henry, for what anyone will say about him willingly seducing another man's wife, actually, like Don, knows how this is done. Betty is new to it and still has romantic notions which were crushed in this episode. Henry cannot show up at her house proclaiming love, bringing flowers, or taking her to a nice restaurant. Sex in an office, in a hotel, or a car, or like Don, lying and showing up at odd hours to a lover's door is all it would be, and she saw that and was deflated. This is the obstacle women have who entertain affairs over men like Don, men accept an affair for what it is (Don even says when the teacher points out the negatives "so what?" its just sex and a release to them) Betty wants romanced and that is not possible for a married woman. How could a man romance a married women where the husband is around? Its a mission impossible for any man. He asks her "I don't know what you want?" because she wants a romantic hero and cheating for a married women is not romantic, it sex in cheap hotels under a assumed name.
Sal also has a lack of power, as his homesexuality blew up in his face. Don told him in the first episode of this year to "limit your exposure" from London Fog pitch. He didn't. That it wasn't his fault is almost beside the point to Don. He was hit on by a very important client and turned him down and it jeopardized the account. This is the storyline that will get the most discussion because Don was very cruel, but his attitude was not unexpected for the times.
The client had total power over Sal, and the fact that Don knew of Sal's homosexuality actually hurt Sal's chances. He guessed (correctly) the client hit on him because he knew he was gay The client could demand anything because it would not hurt him. The guy was a jerk. He knew Sal could not tell on him and he had total power to exact revenge. Sal was between a rock and a hard place. I don't think he will be back, poor sal!