Frankly, the "Life lessons" tag seems rather ironic given the distance between what the lesson the drama thinks it is imparting, and the lesson that it is actually imparting.

The actual lesson is that ML's issue was not choosing almost anyone else besides FL as a spouse. More concretely, he might want to notice how basically everyone, from the less helpful other dads, to the philandering Yoon-ki to his wife's lover, are not cheated on by their spouses (not even Yoon-ki, whose wife simply dumps him after a long time of putting up with his atrocious behaviour, while his wife's lover was, insultingly, taken back essentially immediately).

The take away is clearly that he should have chosen someone worthy of his loyalty and trust, which FL, being disloyal and having broken his trust, is clearly not.

We are instead regaled with, well, nonsense, such as the incredibly insulting notion of an Orwellian thought crime, where having fantasies and feeling attraction, which are perfectly normal, are compared to have a full blown affair, which is a complete and deliberate violation of your partner's trust. One of these things is, self evidently, not like the other.

ML's big fault was not being a mind reader and actually trusting his wife when he asked her if she was okay. He was willing to help, so it's clear that he would have not been against splitting tasks if she felt overwhelmed. He didn't realise something that she didn't realise herself, and then explicitly lied to him about, putting up a facade of everything being just peachy. Her lover found her when she had just realised it and had her guard down, and it was her decision to be completely uncommunicative with her husband, when she had so many other options, from paying for stuff instead of cooking herself, to ask for her husband's, family's or friends' help, to just reconsidering some commitments, such as the pandering stuff in the moms group.

What is certainly not going to help is having an affair with your client. That would not magically open up your schedule, though for some reason she, again, seems to find the time to plan their rendezvouses and book the places they want to sleep at, and buy sexy lingerie. Time dilation? Not sure what universe has such physics. Probably one where the supposed "message" makes any sense.

As an aside, I do find the differential treatment with respect to Yoon-ki rather laughable and hypocritical. Let's be clear: both Yoon-ki and FL were liars and cheaters that were willing to betray and deceive their loyal spouses forever for self serving reasons, without any guilt. FL herself admitted that she would have continued to deceive ML indefinitely had he not caught her red handed, with no consideration for either him or the kid during the affair.

To be perfectly honest, I actually do find Yoon-ki's blatant shamelessness less hypocritical than FL and her lover's calm, polite, apathetic facade of decency. I mean, nothing more irritating than FL's lover politely greeting/introducing himself to ML after the latter had just caught him red handed, as if everyone in the room didn't know perfectly well that had ML not been in that elevator, he would be sleeping with his wife in that hotel room. Equally irritating was the birthday gift: is one supposed to be impressed by that? When she was willing to lie to his face for months, and continue doing so forever? I mean, this is like someone stealing all your money and leaving behind an online shop coupon: adding insult to injury.

The other thing with Yoon-ki was the fact that at least his goals were aligned with his actions: he wanted to sleep with other women, and he did. By contrast, FL's betrayal was not only cruel, but also utterly needless and easily avoidable. It in no way addressed the issue of her overscheduling, as explained above, and she could have very easily just stopped being completely uncommunicative and lying to her husband. The sheer futility is, again, something that adds insult to injury.

To be clear, I do find Yoon-ki absolutely repulsive. But it's a bit like the situation with FL's lover's wife: she could see perfectly through FL, but at the same time apparently didn't realise that her husband was exactly the same kind of person: a liar and a cheater that would have been willing to betray and deceive their loyal spouse forever, and to risk jeopardizing her child's happiness, over a whim.

I also thought that there was a fundamental muddying of the waters where two very different matters are conflated together: unhappiness and betrayal. Obviously, being unhappy does not automatically imply that one would be ready to treat their partner with no loyalty, honesty and respect. To pretend that the two are equivalent is mere equivocation and evasion, and, really, self serving cynicism. So the question shouldn't have been "why was she unhappy", but "why, when push came to shove, she did not care enough not to treat her spouse with any loyalty, honesty and respect". In the case of the cheating wife mentioned in the drama, whose husband had previously a kid out of wedlock behind her back, the answer was pretty obvious: the guy had betrayed her, so she had no reason to show him any loyalty and honesty in return, nor was he owed any. I was 100% behind her, in that regard. This was not the situation with either ML or FL's lover's wife.