Posting this as a general question as it's not specific to me but a few situations I've seen recently.
If a newly married man (first five years of marriage) cheats or attempts to, on dating apps, contacting exes etc, what are the chances he'll change and the marriage can be happy and successful going forward. Let's say he goes to therapy etc and addresses why. Perhaps doesn't drop out of therapy after a few sessions. If the wife can forgive, the marriage was otherwise good, etc. How to rebuild trust? Or how about short affairs in the context of a longer marriage involving kids, another thing I've sadly seen quite often, but where the couple did stay together.
More generally, do these people change? How about compulsive liars, or situations with financial infidelity (both situations I was in where I ended up giving up and leaving). I feel like you have to set clear markers for change in order to orient yourself in the marriage and to know when to leave. Often the cheating / lying / addiction behavior is so ingrained it's almost impossible for that spouse to be different.
If those behaviours don't change, can people really be happy? Can you have a happy marriage long term without trust? I felt I couldn't which is why I gave up, but I have seen women who have remained married for 2 decades and seem happy. I realise it's complicated of course like most things in life