Same sex marriage was not legally recognized in the same way as heterosexual marriage in Rome. For example, it doesn't appear that the moral legislation that governed fidelity applied to it, nor other legal issues like divorce.

Still, there is a reasonable amount of evidence that homosexual couples performed marriage rites. Martial, for example, writes (12.42):

The bearded Callistratus has been taken in marriage by the lusty Afer, in the same way as a virgin is usually taken in marriage by her husband. The torches shone forth, the flame-coloured veil concealed the bride's countenance, and the language heard at bridals was not wanting. Even the dowry was settled. Does not this seem yet enough to you, Rome? Do you expect that the bride should present the spouse with pledges of affection?

However, most of our sources on Roman homosexual marriages are condemnatory (like accounts of Nero's wedding), so while it is definitely plausible that same sex marriages happened, they are also a kind of strawman used to attack homosexuality.

Another piece of evidence that same sex marriage was practiced in Rome is the fact that it was banned in 342. The Theodosian code 9.7.3:

Emperors Constantius and Constans to the people: When a male gives himself in marriage to a "woman" [sc. effeminate man], and what he wants is that the "woman" play the male role, where sex has lost its place, where the crime is such that it is better not to know it, where Venus is changed into a different shape, where love is sought but not found, we order laws to arise, justice to be armed with an avenging sword, so that the disgraced persons who are or in future shall be guilty may be subjected to exquisite penalties.

It has been argued that since this precept focuses more on moralizing than defining any actual legal penalty, it is likely that there was opposition to this measure.