I saw this referred to online and I found the idea that it was accepted in Rome interesting. I had always remembered Romans as being very critical of men who allowed themselves to be the penetrated ones in a relationship when compared to the ancient Greeks (I make the distinction because if I recall correctly the issue wasn't male-male contact so much as allowing oneself to be dominated).
I have found some online sources that say that it occurred during the Principate but I'm wary of accepting it too easily because I know this is a hot button issue. As usually happens when something is so interesting to contemporaries, many activists or crypto-activists like to rewrite the past to legitimize their current positions, so I don't want to be taken in.
Full disclosure: I have no problem with same sex marriage. I just don't want to repeat half-truths or convenient untruths out of ignorance when the topic comes up.
Edit: I am also skeptical of all of this because I remember Augustus and other emperors enjoyed policing public morals, and they viewed marriage as an institution which guaranteed the reproduction of the Roman people (few among the aristocracy were having kids, getting married). Moralizing emperors would therefore probably be displeased with a union that precluded biological offspring.
So I'm restraining myself to one case, because it's illustrative of the problem of same-sex marriages. I don't really work much beyond the Julio-Claudians.
Same sex relationships of course happened. They weren't always popular. Caesar spent time in Asia Minor when he was young at the court of the king Nicomedes. Decades later when he triumphed, his soldiers chanted (in Mary Beard's translation (The Roman Triumph p. 247):
Caesar screwed the lands of Gaul, Nicomedes screwed our Caesar, Look Caesar now is triumphing, the one who screwed the Gauls. No Nicomedes triumphs though, the one who screwed our Caesar.
These chants are meant to be satirical, so you can't tell if Caesar actually had a relationship or in what capacity with Nicomedes from this passage. But you can tell that the soldiers chanting it would think that people would get a laugh out of it.
In contrast, Suetonius writes this about Nero:
He castrated the boy Sporus and actually tried to make a woman of him; and he married him with all the usual ceremonies, including a dowry and a bridal veil, took him to his house attended by a great throng, and treated him as his wife. And the witty jest that someone made is still current, that it would have been well for the world if Nero's father Domitius had had that kind of wife. This Sporus, decked out with the finery of the empresses and riding in a litter, he took with him to the assizes and marts of Greece, and later at Rome through the Street of the Images, fondly kissing him from time to time.
Suetonius is notorious for for scandal mongering and you have to be very careful about using him as a historical source. But this does point out that the concept of male same sex marriage was something imaginable at least to Suetonius, or his gossiping friends.
As a corollary to these anecdotes, the jurist Gaius (writing around the time of Suetonius), notes an oddity of Roman marriage law. A man is allowed to marry his niece, but a woman is not allowed to marry her nephew. The reason for this is any act an emperor does is inherently legal, and Claudius married his niece, so that is allowed. If that legal logic holds, there should be legal man-on-eunuch at least marriage. But I've never seen that anywhere spelled out. (Sorry, I don't have Gaius around at the moment).
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