Well, the three 18th century Royal spouses who as far as we can know never cheated on their wives were: 1) FW (no comment necessary), Louis XVI. (might not have needed sex tips from his brother-in-law otherwise, she said flippantly, but seriously now, no mistresses for Louis, which did not make Marie Antoinette a happier Woman), George III (was by far the best husband of any Hannover, though that's damming with faint praise, given how rotten the other Georges and William were, but otoh: Farmer George did go mad).

All of which is not meant to excuse Franz Stefan cheating on Maria Theresia, but to observe sexual fidelity wasn't the ultimate criterium was to which royal marriages were successful ones.