Now thats not true.Am I crazy to remember Roxy not knowing he was married yet in the LNs?
Also, just to be clear on the cheating outrage: there's plenty of degeneracy in this series, to be sure, but do keep in mind that Sylphie explicitly told Rudeus that she'd accept him having a second wife (at least in the LNs). Rudeus knows that she won't get mad. Monogamy is somewhat uncommon in that world.
Sure polygamy that is not uncommon, but more or less monogaminis quite the majority especially because Milis religion has a kot of influence around the world.