Yes, this is the whole point of Dianne in my opinion, to show Lacey that even if Her and MC enjoy something in the moment, that it can actually cause real damage, similar to drugs. As Deviant said, MC agreed to it (even if Lacey did give reasons for why it should happen). MC couldn't stop orgasming after it, as far as I'm aware, it's the only time since their wedding night that their sex is described like that. Lacey straight up says it's the best sex she's ever had. He obviously enjoyed it in the moment, and so did Lacey.
But then MC wakes up to nightmares
It's one of the things that frustrates me when people complain about Lacey in particular. Lacey gives reasons for her actions, but the story does not forgive her actions. Even the jealousy date through MC's nightmares and Diane's conversation is shown to be harmful. This story says that Lacey's actions are harmful to both herself and MC, and she is currently coming to understand that and trying to improve.
This isn't the story of a girl who had a bad past but has moved on from it. This is a story of a girl who has made no real effort to recover from her trauma until the end of Act 1. Act 2 is really what I consider the start of her recovery, so all of Act 1 was her having everything she wanted, but nearly losing it all due to the trauma still consuming her life.
Now, when it comes to Mia, I can agree to a point. The story has not done much at all to show her actions in a negative light. However, I personally think it's because there was so much that Lacey needed to get through that Mia's storyline is saved for another act. We only have 2 acts so far. Give it time. TV does this all the time where it will focus on developing one character at a time for an Act
Agreed on Lacey getting better, but she is still way off the mark, I think you give too much credit to a couple of things.
MC is extremely angry during the "make up sex" of the jealousy date, the author paints it as a fantastic thing but because MC is painted as a fantastic cuck there.
It was worse than the punish route in terms of aggressiveness and it was obviously a sign that, no, everything was not alright.
MC wakes up to nightmares because he just experienced his nightmare first hand and he had a part in it.
Ok we cannot ignore the million hints about MC being a cuck here and there, in both slut and no fun path, still I think it is a trait born from inadequacy, sexual inadequacy to be exact, which is actually how the kink is born in normal people.
Lacey faces literally NO consequences ever, no matter what she does or what happens, nothing ever changes.
Not her relationship with the friends (maybe the crew could be an indication but it seems a group born with other purposes), not her relationship with MC, not her work, nothing.
Getting shouted at a few times by MC so he can be satisfied and retreat to his timeout cage is not exactly a consequence.
After Damian the dynamic should have shifted, hell, even after the atrocious 5 boxes dialogue, yet nothing happened.
Her close friends still love her, her job position is stronger than ever and Mc is properly leashed and never strays.
The pimp is even worse, she doesn't even have the excuse of brain damage, she does heinous thing after heinous thing without ANYONE ever telling her off.
Lacey: you almost destroyed my marriage and gave hurtful information to my husband out of spite? (remember this is the original plan)
nothing
Anna: Oh you used hateful and disgusting language to try and hurt the man I love? bad girl! I will not talk to you for one week and then I will apologize
MC: Oh you did all that and then tried to break my marriage again by bringing harmful drugs to my addicted wife? Don't worry I won't be angry with you.
The reality of things my dear friend, is that there are no consequences for female characters in this game, not for Lacey, not for the pimp, not for Veronica, not for christine and not even for Anna (I am disappointed is all she gets).
This is why we keep on seeing new big bads, because there can't be progression in the relationships caused by the events of the game.