"So many people take the moral high ground."
I can't imagine using 'moral' as an insult.
I think 'once a cheater, always a cheater' holds true because they have shown themselves to be capable of those behaviours and traits necessary to conduct an affair.
People who say that they have cheated but never would again are fooling themselves imo. You might be deliriously happy with your current partner but, should they ever begin to make you feel like your old partner did, you would be tempted to cheat.
It's like any bad behaviour. The first time is hard, but once you've done it once it's easier, less guilt-inducing, to do it again.
I'm sure some cheats go on to marry ow/om and be very happy. But I know several couples in this situation - they give the appearance of great happiness but privately admit to insecurities and mistrust. One friend said she has sex with him whether she wants to or not, because she knows lack of marital sex led to their own affair. Another, in the same conversation, said she puts up with certain behaviours and backs down from justified arguments, because 'we'd look like idiots now if we couldn't make it work.'
I don't think that all cheats are bad people. I understand that there are many different reasons for affairs. But the ability to lie convincingly, to maintain everyday life with a spouse while conducting an affair, to prioritise time with an affair partner over other commitments, not skills that everyone is capable of.