"They are wives."
Translation:Loro sono mogli.
I was thinking that Italian was globally rational and logical
That's not how language works. Language is a natural phenomenon. Rationality and logic have nothing to do with it. Some languages are more regular than others, but no natural language is perfectly regular.
Yes, for the most part, native Italian nouns tend to follow the pattern
But as you have discovered, this is not always the case. Sometimes nouns will be
and you need to memorize on a case-by-case basis whether they are masculine or feminine.
Foreign loan words often end with different sounds (often consonants) and tend to be masculine.
Italian has taken the '-us' ending of a good many masculine Latin nouns (nominative case), replacing it with '-o' in the singular, but kept the '-i' ending of these nouns in the plural. Likewise, it has kept the '-a' ending of many feminine nouns in the singular, and deleted the 'a' from the '-ae' ending of these nouns in the plural. It has also scrapped the neuter gender but subsumed most of these nouns into the masculine gender.
This is somewhat of an over-simplification, but basically correct.
Actually, the accusative became the standard form in Romance languages. The 'm' in all um (by this time -om), am, em forms became nasal (õ, ã, ~e) and then o, a, e.
However, confusingly, the nominative was taken in the plural (ae>e, ī>i, es got replaced by ī>i).
Wikipedia has a great explanation, step by step:
I can imagine it as a category. Like "Loro sono mogli" because they behave like wives, more than women, more than mothers, more than whatever other "women category".
It sounds a bit strange, but that's the best interpretation I have.
Anyway, if you find sentences whose context is unclear, it could be worth reporting it to Duolingo, but I don't think they will change their data base. Maybe you can suggest a similar pair "English sentence-Italian sentence" which makes more sense.
There might be some confusion because "loro" is both the personal and the possessive form. Only the possessive form would ever take "the" -- il loro x; i loro x; la loro x; le loro x. Loro as the subject of the sentence has nothing to do with whether a separate noun phrase would take "the" or not.
It's true that most of the time, this pattern holds:
-o masculine singular
-i masculine plural
-a feminine singular
-e feminine plural
But some nouns are irregular. They are -e in the singular and -i in the plural and you need to memorize each individually whether they are masculine or feminine. This is the case here:
la moglie (singular)
le mogli (plural)