The problem here is your assumption that monogamy is the absolute relationship and anything else is invalid, their are a growing amount of polycules that exist, polyamorous relationships, what is 'normal' is very subjective, while yes the MC might have perceived things from that could be deemed 'normal' for most of us to begin with, it doesn't mean over the course of the story he isn't being changed by everything that is happening. Also you fail to take into consideration the effect of Lacey's influence over him, if he is told repeatedly that doing certain things are fine, in time he'll come to believe it.
Considering the power Lacey hold over him do you think he wouldn't in time come to accept her claims? That his resistance wouldn't end up eroding away? Ever heard the expression sink or swim? Well the MC had to learn to swim thru Lacey's currents, or sink, he has tried not to go along with the flow completely but he had no choice but to somewhat still follow the flow, as I said earlier its about the necessity of compromise in relationships, obviously he is compromising far more than Lacey, but that fit the character.
Going back to 'normality' did you know prostitution is common in High Schools in Japan? To us here it would be scandalous and horrible, but over there its fairly common for a girl to be paid for sexual activities in High School. Different norms entirely, what many consider here to be absolute norms, aren't by any mean absolute, similarly you had plenty of civilization where polygamy existed and did society crumble under the weight of depradation? No.
For individuals like the MC and Lacey that basically had to some degree self teach very likely much of what could be deemed the norms they had likely a very high degree of capacity for adaptation, in particular for the MC which grew to react to Lacey's issues perpetually. Likely the MC only learned 'normality' in College, while Lacey didn't even because she went crazy wild instead. If the MC only learned 'normality' as an adult its potentially far less ingrained than it would be in most of us. Which might be the source of much of Lacey's issues, her normal is likely what she developt in College, we know what happen in College, so it mean to her fucking around and such is normal, which is why she doesn't see the issue with having sexual activities with other people as problematic, because its her normality.
Sure likely after marrying the MC she had to learn to take some notion of what he deemed normality, but just like it would be quite particular for any of us Western folks to be marrying an Indian, well it doesn't mean she lost her sense of her normal, merely that she had to reign it in for the MC, and thru drugs, alcohol, personality issues, etc she is struggling with upholding that normal the MC imparted upon her, and she has been trying instead to teach him her normal.