[Ziusudra]

Do cheating married men ever tell their 'mistresses' or 'temporary sexual or emotional flings' that they are in a happy marriage??

The rate of cheating married men marrying their mistresses is less than 5% - according to a British study that I read about several years ago.
And the failure rate of those marriages is extremely high, like in 95%.
Yeah, so the British prince who married his long time mistress is an exception.
But their relationship history is atypical than other 'affairs'.

Instead of being an enabler for this troubled married man, tell him to get some marital counseling ' WITH HIS WIFE'.
He needs a professional 3rd party to listen and to help him, NOT you who obviously developing romantic feeling for the man.

You may also benefit from therapy to figure out why you are focusing your energy on a married man.