I have always thought that Tom was expected to marry a "lady", someone with class who is a good accessory to him. Yet, he and Daisy do not seem to interact-and yet there are limits to how he can treat her.
Because Myrtle is of a different class, he feels like a king (in her eyes, especially), he can verbally berate her, physically assault her, and yet it does not affect him as it would if he treated Daisy that way, because Myrtle is just his mistress.

While Tom and Daisy are members of the social elite and upper class in the novel, Fitzgerald wants the reader to be aware that they are not morally elite or upper class - they are actually immoral and selfish people. Tom's attraction to Myrtle is a reflection of this. Myrtle is a lower class person and also immoral. But she differs from Tom in that her physical appearance and social standing reflect this. Tom is attracted to exactly what he is - an immoral, lower class person. Daisy's attraction to Gatsby is actually an attempt on her part to reject her immorality and become a better person. However, because she ultimately spurns Gatsby and therefore her chance at becoming a better person, in the end she destroys her chance to achieve goodness. That's why she leaves with Tom in the end. Daisy and Tom are both morally bankrupt people who deserve each other. Gatsby is too good for Daisy. However, Myrtle is not too good for Tom. It is Myrtle's desire to use Tom in a selfish way to try to improve her position in life that leads to her destruction. Myrtle is not a good person either - that's why Tom is attracted to her.

Excellent posts! The reader can tell that Myrtle is trying her best to play the part of upper class in the way she puts on her best dress and tries to cordially entertain guests at the apartment she and Tom own. We're led to believe that we may have had a deeper glance into Tom and Myrtle's relationship the night at the apartment, except our narrator got drunk.

"Likes" is probably too strong a word for the way Tom feels about Myrtle. Myrtle is an easy target for the abusive Tom. Myrtle, aspiring to a life beyond that of her working class husband, accepts his abuse. Tom takes a sadistic satisfaction in feeling superior to Myrtle (as he cannot to Gatsby or Daisy) and enjoys taking out his shortcomings on someone weak and needy.

Myrtle feels inferior in many ways. Sexually, she knows she is not beautiful like Daisy. (Nick describes her as carrying her "her surplus flesh sensuously as some women can.") She knows she is out of her league chasing Tom and the social circle he revolves in, but hopes desperately and pathetically to fit in.

I've always believed that Tom likes Myrtle because he can impress her. With Daisy, nothing Tom does is new, or inspiring, she's bored with him, they're bored with each other. Myrtle, however, worships Tom, and treats him like a hero (which is how she looks at him).