New ex Jw's are like a kid in a candy store with no parents around..they go wild and often end up with a tummy ache from all that "Candy"...
Some are. Some aren't. Not a one-size-fits-all world.
About those marriages that broke up: well, more than half of all marriages break up. With or without extracurricular activity. At that point or years after. You can't generalize. I'm sure it happens, but it's not a rule.
Open relationship: something like 7 of my 23 years together were open. They came in the middle of the relationship rather than the start or the end. For me impersonal sexuality outside of my primary relationship worked ok.
Polyamory did not work for me. My emotional make-up is such that I do not enjoy maintaining deep sensual connections with more than one person at a time. I am serially committed in this regard.
Swinging isn't for me at this stage in life, I have little time or energy to hop around or play within any particular rule set, which most of the swingers I've known seem to like to set up. Your mileage may vary.
In my experience, along with those I've known for decades, the only rule is to keep agreements clear and abide by them. There are no rules beyond what consenting adults consent to. Having agreements sets boundaries for physical and emotional safety; following those agreements allows trust.
Be safe...have fun...don't dream it, be it.