My question is simply stated: In Roman classical antiquity, either by legal arrangement or even anecdotally, is matrimonium ever used to refer to a (1) polygamous, or (2) homosexual relationship? If so, I am especially interested in uses of this term that imply some form of legal or cultural recognition.
I found two references in Aulus Gellius, one of matrimonium and one of the verb nupta.
Tum puer matre urgente lepidi atque festivi mendacii consilium capit. Actum in senatu dixit, utrum videretur utilius exque republica esse, unusne ut duas uxores haberet, an ut una apud duos nupta esset. (1.23.8)
Then the boy, because of his mother's insistence, resorted to a witty and amusing falsehood. He said that the senate had discussed the question whether it seemed more expedient, and to the advantage of the State, for one man to have two wives or one woman to have two husbands.
Mulieres fere omnes in maiorem modum exosus fuisse dicitur, siue quod natura abhorruit a mulierum coetu siue quod duas simul uxores habuerat, cum id decreto ab Atheniensibus facto ius esset, quarum matrimonii pertaedebat. (15.20.6)
He is said to have had an exceeding antipathy towards almost all women, either because he had a natural disinclination to their society, or because he had had two wives at the same time (since that was permitted by a decree passed by the Athenians) and they had made wedlock hateful to him.
(English is Rolfe's revised translation from the Loeb.)
Tacitus in his Germania 18.1 also mentions matrimonium in the context of polygamous barbarians, but it's a more tenuous connection, since he's contrasting the Germans' adherence to monogamy against others' practice of polygamy.