In fairness to Sark, Cécilia was a whore who cheated on and left him. So if he cheated on her too, big deal.
Sark may be not be a saint but Hollande sauce seems a bit higher on the sleaze/soap opera scale. Running against his ex for the nomination, beating her and then making her President of the Parliament? This would be Bill and Hillary if only America cared as little about family values as France seems to!
Just because she cheated doesn’t make it okay, morally, for him to do the same. The principle is still the same. Divorcing her would have been the way to go.