I agree with most of that actually. But there are a few things I want to pick up on.
If you’re unhappy you have the right to leave a relationship. But you do not have the right to trample over the innocent parties left behind. I.e. the children.
Well that's easy to say and less easy to do, isn't it? How do you exercise your right to leave without automatically trampling on the children's feelings? Very, very few children of any age are going to be completely emotionally unaffected by your decision. Especially if the decision is not mutual and they have to witness the other parent deeply hurt and financially inconvenienced by your decision. Your kids are going to probably going to feel 'trampled,' no matter how sensitively and reasonably you think you've tried to handle the situation.
That's just the price you'll have to pay for daring to put yourself first. Parents aren't supposed to do that, ever. It's not what we told our children they could expect from us, is it? They expect parents to supply an unending stream of selflessness and martyrdom and no horrible shocks. Obviously, despite the promises we all make to our children and the things people tell themselves, parents do sometimes make selfish decisions, big or small, that affect or damage children all the time.
Sometimes they genuinely don't see it and other times they know very well what they are doing, but they do it anyway because they think their own feelings are too big to ignore. So they apply cognitive dissonance to make themselves feel better about what they are doing.
You choose to leave your wife and children for another woman? Don’t be surprised when those children want nothing to do with that woman.
Absolutely agree and have never tried to suggest otherwise. It sounds to me as if this man has completely taken that on board. He keeps a good relationship with his girls while accepting that they don't want his OW in their life. Will they be happy to continue like this if he says he's getting married? Maybe, maybe not. He'll have to factor that in before he makes his decision.
Nobody has the right to happiness at the expense of others.
Well that's patently untrue. We have no control over how other people will feel about the choices we make for ourselves, but it doesn't mean we don't have the right to make those choices.
We should consider how our choices will affect others whose happiness we feel at least partially responsible for, and weigh up how we should proceed based on that. And obviously that should never be more true than with our children. But with everybody else, including spouses, we have to be free to consider our own needs too. Otherwise we'd all be forever trapped in relationships, friendships or unhealthy family dynamics that are detrimental to our own happiness and wellbeing.
If we inadvertently hurt other people in the process of making ourselves happy, that's an awful shame but it's sometimes unavoidable. Anyone who has ever had to break someone's heart knows this. It doesn't mean we shouldn't be free to do it anyway.