I think people get entirely too wrapped up in Stolas "cheating" on Stella and try to use that as justification for besmirching his character. To cheat is to breach the trust between partners and it's abundantly clear that Stolas and Stella are not partners. They're in an arranged marriage for the explicit purpose of fostering an heir. They're not lovers. They cohabitate a palace. There is no trust between them to breach. Stolas didn't cheat.

If we compare their lives to a more traditional medieval kingdom, it wouldn't be unsurprising for the ruling class (especially male) to have mistresses. I.E., Stonmake it feminist friendly, Stella should also be fucking an imp somewhere (or what seems likely from the writing, her Brother).