Like everyone had mentioned, lakorns and dramas are for entertainment, which may contain 0 to 100% of reality, just depends on whose reality it relates to. I honestly cannot say what the main theme of this lakorn is but these are my guesses and usually, the majority of lakorn themes are, good people triumphs in the end.
Let's not forget, the primary reason as to why Pai married Kong Kai and is willing to endure humiliation, disrespect, and misery is to protect someone she dearly loves, Khun Ying. When you're in a relationship for certain reasons, other else than love, you become numb to all of the condescending situations Kong Kai has placed Pai in because there's no love, so there's no passion, no jealousy, no desire, overall no emotions. In contrast, P'Choke, Pai's ex-boyfriend, someone who she actually loved and broke her heart, she didn't allow him to continue cheating on her and clearly drew the boundaries. The saying goes, the more you love, the more you hurt, the more you hate and in this case, whatever Kong Kai does, does it bother Pai? Yes, because she's human, she feels disrespected but she has always stated, disrespected from a woman's point of view, not exactly from a true wife's or lover's standpoint. Pai would rather continue protecting Khun Ying, putting aside her feelings and pride, rather than following her emotions and fail the mission from the beginning. There's just been too much investment into the marriage, the stakes are high (Khun Ying's life), and it's not worth it to sacrifice everything just to yield to Kong Kai and his attacks.
In Dok Som See Thong, both wives loved their husbands, quite a different situation from Mia Thaeng.
I think the whole theme of Mia Thaeng isn't about how it's okay for a husband to cheat all he wants while the wife remains pure and loyal at home working and cooking. I think the main theme of this lakorn is like what was said in the lakorn, there are two different types of women, they're like cars, one type of women are like leased/rented cars where you don't dare show off to your friends saying it's yours no matter how extremely expensive and gorgeous the car may be because it's rented/leased, even you don't know how many drivers have sat in that used seat versus another type of women who are like bought cars, you're the first and true owner, you know how it's like because it's yours and you can show it off to friends and family.
The lakorn is teaching us, a woman you should marry should be like Pai, someone who has a load of patience, a good heart, can be very independent and career driven, has her own life, yet be gentle and domestic, someone who will love you not based on your wallet but your heart, someone who is willing to sacrifice their life for someone else's (Khun Ying), someone who doesn't sleep around, who is simple and kind, who has self-respect and pride, and someone who you would also end up truly falling in love if you took time to examine long enough without jumping into the sheets first. We have to admit, men these days would love and choose Broong for all of the wrong reasons because she satisfies them momentarily. I mean, she's good in bed, sexy, hot, popular, loves to party, wild, keeps men on the edge, which blinds men from her true ugly self, she's manipulative, materialistic, whore, selfish, and rotten. There's a deep comparison between two types of women, Mia Thaeng emphasizes the old traditions and morals for what makes a good wife versus someone who you shouldn't marry because she's nothing but bad karma. It's teaching the audience, don't judge a book from its cover, pretty things on the outside may not be so pretty in the inside. Pai is all of the above, beautiful, good-hearted, and a true gem, if only men are willing to use their hearts to find love and marriage rather than their little brothers. Then perhaps divorces will lessen because marriages would be formed from true love, the right beginning, and not from lust and other wrong reasons. There's another saying, behind every successful man is a successful woman; it's the successful woman that pioneers the man towards success. Women really can influence and guide their husbands to the right directions.
Now, I'm not saying that every guy who marries Pai will never cheat. I know some men who have married amazing women yet still end up cheating but that's because some men will never see the value in women like Pai because they're rotten themselves, so they don't deserve such great women and will in the end lose the best thing that ever happened to them and will eventually fall into an endless pit of lust, sin, and misery, but that's a whole other story that Mia Thaeng doesn't necessarily touch basis with. Khun Yai in Dok Som See Thong cheated on Dee, who was very classy like Pai. But like Pai and Dee, you don't need a man to up your worth, it's within yourself.
As for the main theme towards women, it's about, respecting yourself. Men in the end want wife material women, they will only use you and dump you once they've found a Pai of their own. LOL Girls like Broong are wanted now because they're still young and hot, but once they grow older and beauty fades with age, there's no substance to their character, so they can only attract but never keep with such shallow qualities.