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My experience: I was 19 and away at school in my second year of University when my dad unexpectedly informed us all that he was leaving my mother and moving out. My younger sibling and I (senior in high school) were informed that he had been unhappy for a long time, and life was too short to stay in an unhappy marriage. We were told he would be living alone. 6-8 months later, my sibling inadvertently discovered (via coming across his car parked in a strange location) that my dad was in fact, living with another woman. I was never sure how much my mom knew or suspected, but she either took the "high road" (as you viewed it) or kept my father's secret for him, until it all blew up.
When my dad originally left, I was upset, of course, but no part of me felt entitled to know the particular details of exactly what was going on. I knew my mother was upset and blindsided, but I mostly took what my parents said about why at face. I had never really observed my parents marriage to be particularly happy - not horrendous or abusive or anything - just not in what I grew up thinking being "in love" would look like. He didn't seem to have a lot of respect for her, and she seemed to have a lot of resentment for him (as it turns out, because he had been cheating on her for years, so that explains that!). Based on what I'd observed about the marriage- I didn't ever resent either of them for not trying harder to fix things. I never begged for them to try to get back together - in my mind, that is what little kids (who worry more about how things effect them, than about the wellbeing of their parents) might do.
When I found out the truth however, I was mad at BOTH my parents for lying to me. I felt like I was old enough to know the truth and to not be treated with kid gloves. So yeah, I think that the truth is important - obviously in an age appropriate way. I'm not sure why the particulars would be necessary however. My anger for my mother for not treating me like an adult was clearly overshadowed with my anger for my father who was clearly living a lie. Seeing text messages or photos as proof, was absolutely not necessary for me to get to anger. In fact, they would have been horrific and unnecessary. I do remember my dad trying to justify his actions to me based on his perception of a dead bedroom** and my mother feeling the need to assure me she had in fact done her very best to ensure that the bedroom was not dead , and if I could excise those details from my brain, I assure you I would. I promise you, no 19 year old, wants or needs to know the details of their parents' sex lives.
Mindracing - it's not clear to me how old you were or whether you were told the truth about your mother's A at the outset. It sounds a bit like you weren't or that if you were, your mother convinced you that whatever she did should have been forgivable and you held this against your father. You say you don't think explicit details are necessary, but that more are necessary. What more are you advocating for?
I think it is belittling to say that showing them evidence will irreversibly harm your children's psychological development. If this is the worst thing they ever see / experience, I would call them lucky.
The pre-frontal cortex of the brain does not reach maturity until well into a person's 20s - some estimates suggest mid-late 20s. So yes, while they are legally an adult, their brain has not stopped developing. I think your parents' divorce is going to impact you psychologically no matter when it occurs. Most kids grow up respecting and identifying with one or both parents to some degree. Learning that that person is "BAD" is going to call to question a lot about what you thought you knew about yourself and your childhood. I don't see why any parent, no matter what the kids age, should be lackadaisical about their kids' mental health. Just because some people have experienced worse, doesn't mean that it is easy or that they are lucky. I think when giving details, a parent should consider what the purpose of providing that information is. If the kid is legitimately struggling to make sense of something or is making incorrect assumptions based on the limited information provided, particular details might be appropriate, but if the goal is simply to convince the child that the other parent is the one at fault, I'm certainly more skeptical. Having a healthy relationship with both their parents, is generally in a kid's interest, whether that kid is over or under the age of majority.
** To be honest, hearing my father attempt to justify his infidelity on this absolutely did damage to me. Can you imagine as a young woman- yes legally an adult, but still a kid in many ways, and certainly naive about how grown-up relationships worked - being informed by your father that, in his mind, a woman that didn't "put out" deserved to be cheated on and abandoned? That is absolutely something I internalized.