Socrates Historicus, 4.31 – Valentinian’s Two Wives
“When the empress Severa saw Justina bathing, she conceived a desire for the maiden’s beauty. She talked to the emperor about her, telling him that the beauty which the virgin daughter of Justus possessed was so incredible that even she, the empress, (though she was a woman) was smitten by desire for her. The emperor, weighing the empress’ speech, decided to wed Justina while retaining his current wife, who had given birth to Gratian (whom he had just declared co-emperor). He therefore issued a common edict throughout the cities, allowing anyone who so wished to have two wives.”
῾Ως οὖν ἴδεν αὐτὴν λουομένην τὴν ᾿Ιουστίναν ἡ Σευήρα, ἠράσθη τοῦ κάλλους τῆς παρθένου· καὶ πρὸς τὸν βασιλέα διεξῄει περὶ αὐτῆς, ὡς οὕτως εἴη θαυμαστὸν ἔχουσα κάλλος ἡ παρθένος ἡ τοῦ ᾿Ιούστου θυγάτηρ, ὡς καὶ αὐτὴν, καίτοι γυναῖκα οὖσαν, ἐρασθῆναι τῆς εὐμορφίας αὐτῆς. ῾Ο δὲ βασιλεὺς ταμιευσάμενος τὸν τῆς γυναικὸς λόγον, ἀγαγέσθαι τὴν ᾿Ιουστίναν ἐβουλεύσατο, μὴ ἐκβαλὼν τὴν Σευήραν, ἀφ’ ἧς αὐτῷ Γρατιανὸς ἐγεγόνει, ὃν μικρὸν ἔμπροσθεν ἀνηγορεύκει βασιλέα. Νόμον οὖν ὑπαγορεύσας δημοσίᾳ προτίθησι κατὰ πόλεις, ὥστε ἐξεῖναι τῷ βουλομένῳ δύο νομίμους ἔχειν γυναῖκας.
That old fuddy-duddy Edward Gibbon tries to dismiss this story as fanciful:
“But we may be assured, from the evidence of reason, as well as history, that the two marriages of Valentinian, with Severa, and with Justina, were successively contracted; and that he used the ancient permission of divorce, which was still allowed by the laws, although it was condemned by the church.”
-Decline and Fall of the Roman Empire, Vol. II Chp. 25