Nero, the Roman Emperor, married a man, one of his freedmen taking the role of the bride.
According to controversial historian John Boswell, what is arguably the first historical mention of the performance of same-sex marriages occurred during the early Roman Empire. These were usually reported in a critical or satirical manner.
Child emperor Elagabalus referred to his chariot driver, a blond enslaved person from Caria named Hierocles, as his husband. He also married an athlete named Zoticus in a lavish public ceremony in Rome amidst the rejoicings of the citizens.
The first Roman emperor to have married a man was Nero, who is reported to have married two other males on different occasions. The first was with one of Nero's own freedmen, Pythagoras, with whom Nero took the role of the bride. Later, as a groom, Nero married Sporus, a young boy, to replace the adolescent female concubine he had killed and married him in a very public ceremony with all the solemnities of matrimony, after which Sporus was forced to pretend to be the female concubine that Nero had killed and act as though they were really married. A friend gave the "bride" away as required by law. The marriage was celebrated in both Greece and Rome in extravagant public ceremonies.
It should be noted, however, that conubium existed only between a civis Romanus and a civis Romana (that is, between a male Roman citizen and a female Roman citizen), so a marriage between two Roman males (or with an enslaved person) would have no legal standing in Roman law (apart, presumably, from the arbitrary will of the emperor in the two aforementioned cases).
Furthermore, according to Susan Treggiari, "matrimonium was then an institution involving a mother, mater. The idea implicit in the word is that a man took a woman in marriage, in matrimonium ducere, so that he might have children by her."
In 342 AD, Christian emperors Constantius II and Constans issued a law in the Theodosian Code (C. Th. 9.7.3) prohibiting same-sex marriage in Rome and ordering execution for those so married. Professor Fontaine of Cornell University Classics Department has pointed out that there is no provision for same-sex marriage in Roman Law, and the text from 342 A.D. is corrupt, "marries a woman" might be "goes to bed in a dishonorable manner with a man" as a condemnation of homosexual behavior between men. In any case, he points out that the emperors did not like what was going on.