Same-sex relationships, unions and even rituals for said unions have existed in history. Could you provide an example, however, where the union was indeed regarded in the same light as a heterosexual marriage, and there was no real difference between the two? I don't believe this exists (I could be wrong). In Roman law, for example, marriages only had legal status if they were between a man and a woman, though same-sex relationships did exist, notably Emperor Nero had a big public ceremony with all the grandeur of marriage with a man, however, in Roman law the view remained the same as to what marriage was from a legal standpoint.
In the case of Nero, he was the Emperor; his word was law. So, at least presumably, his mere say-so would have made his marriage to Pythagoras (the freedman) valid in law. I don't think anyone would have dared to declare the Emperor's marriage illegal either.
Hopefully you aren't a source snob and say "because it's Wikipedia it's automatically invalid!"
From Timeline of same-sex marriage - Wikipedia, the free encyclopedia
Practices and
rituals for
same sex unions were more recognized in Mesopotamia than in
ancient Egypt.
[5] In ancient
Assyria, there was considered to be nothing wrong with homosexual love between men.
[6][7][8][8] The Almanac of Incantations contained prayers giving
equal standing to the love of a man for both a woman and a man.
[9]
At least two of the Roman Emperors were in same-sex unions; and in fact, thirteen out of the first fourteen Roman Emperors held to be bisexual or exclusively homosexual.
[10] The first Roman emperor to have married a man was
Nero, who is reported to have married two other men on different occasions. First with one of his
freedman,
Pythagoras, to whom Nero took the role of the bride, and later as a groom Nero married a young boy, who resembled one of his concubines,
[11] named
Sporus.
Adolescent emperor
Elagabalus referred to his
chariot driver, a blond slave from
Caria named
Hierocles, as his husband.
[12] He also married an athlete named Zoticus in a lavish public ceremony in Rome amidst the rejoicings of the citizens.
[13]
End source
So some civilisations have actually recognised gay marriages in equal standing as straight marriages at at last some point in history. I hope this is satisfactory to you