I translated this as "That's how he met his future spouse" which is perfectly correct in English. The program said I should have used "wife" even though there is a specific word for "wife" in Italian: moglie. Shouldn't either one have been accepted?
It is an outdated word not so often used now. In the context of the sentence in Duolingo It means "as a result of" or "therefore" or "so". (I could have written "It is an outdated word thus not often used now" and it would work but sounds old fashioned).