Also, WHICH society? SOME societies adhere to polygyny or polyandry. I bet, wait, I KNOW he thinks those are immoral, yet those societies think the are moral. In MANY Middle eastern societies a woman wearing a halter top is a stone-able offense because it is deemed by that society as "immoral."In order for that change to happen, there must first be dissent and opposition to the established norms already in place. That means that at some point someone, or a group of someones must say "No, this norm is not for us, we do not subscribe to it", and this is the hole in your entire argument. Society morals does not equate to 100% consensus by the people within that society, so disagreement to those supposed morals does not equate to immoral.
You dont want to talk about segregation, and interracial marriage because even you know that those "Social Morals" were not "Moral" at all, and the people who went against them were NOT immoral people.
Deliberate injury to a person, and animal, the environment...those are immoral. Is cheating deliberate harm? Well, IMO yes because one has agency and the ability to control oneself and not cause harm.
In the case of the LW I think he's probably cheating and not being straight with LW, but that's my impression.
If OTOH his wife is terminal and she knows about it and consents to it, then it's not, IMO. But, there are too many variables in the lives of not only LW and LW's BF, but in everyone's lives to throw a blanket over it all and say "immoral" which is what he's trying to do, which is why he is (surprise, surprise) wrong.