He wants to take MQQ in as a sister but due to certain circumstances, he was morally pressured to take her as a concubine. After talking to others and understanding how the FL felt, he confronts MQQ and says he won't marry her. FL comes to see MQQ as a sister and ML's grandma adopts her later on. FL realizes that MQQ is not at fault, and MQQ had her own redemption and marries her childhood sweetheart.

He goes from not recognizing FL dressed differently, to finally painting her face as her final test for him.

MC tries to move on with the 2nd ML, who is more powerful than ML. She is special to 2nd ML because she's the only one who can turn him on. But she realizes she feels weak with him, and he only sees her as a woman. Only the ML saw her the way she saw herself, and made the effort to let her be happy, rather than conquer her. 2nd ML can't answer whether he would only be true to the FL is another woman special to him showed up in his life, and blames the ML for setting high standards. However ML promises FL that no matter what destiny plans for them, he will only have her as a wife.

in short, ML's grandparents were the former Prime minister and wife, but because the current Madam Gu (who btw is a second wife and not blood related to ML) disapproved of ML's mother the parents became estranged, even going so far as to separate and move out after the grandfather died. Add on the fact ML's dad could never get close to ML due to ML being unable to recognize his own father, and ML had virtually no practical emotional education apart from the few bits the mother and aunts could impart.

This matters because when ML meets Mo qiqi (essentially the other woman, who in any other story would be the destined female lead) and recognises the first face in his entire life, he can't help himself from wanting to stick around this person. HE originally just wanted to make mqq and her brother righteous siblings, but

he panics and offers to take Mo Qiqi as a concubine, despite never actually learned what it means for a woman to be a concubine.

None of which excuses the way ML takes FL for granted, or how he's never had to take her emotions seriously since she would take care of all the emotional burdan of the relationship. It does highlight that ML is an idiot when it comes to interpersonal relationships, and he does suffer from it for a good majority of the novel as he is forced to fend for himself after FL leaves and is no longer there to cover up his shortcomings

with only a couple exceptions, the cousin and 2nd male lead

a random regression/reincarnation system trying to make its host the new FL by giving her the golden finger of the ML seeing her face.

The FL/wife is incredibly devoted while the ML/husband is a possible cheater, the ML has face blindness and CAN regonize facial features like a beard or their clothes, but can't actually recognize their face, this includes his wife, and so far, the only exception to this rule is the cheating partner (assumed), so while the story hasn't expressly said it (or I just didn't notice), it is believed that the ML cheated on the FL, and as things stand, there are no other male characters that appear directly so the husband is still possibly the ML.