Plato’s symposium speech by Pausanias. Two of the characters in Plato’s Symposium are Agathon and Pausanias who are lovers. Their relationship started when Pausanias was 18 and continued for at least 12 years to the time of the writing. Pausanias condemns exploitative relationships with boys and argues that it’s better to have a relationship after the first signs of manhood.
But the other Love springs from the Heavenly goddess who, firstly, partakes not of the female but only of the male; and secondly, is the elder, untinged with wantonness: wherefore those who are inspired by this Love betake them to the male, in fondness for what has the robuster nature and a larger share of mind. Even in the passion for boys you may note the way of those who are under the single incitement of this Love: [D] they love boys only when they begin to acquire some mind—a growth associated with that of down on their chins. For I conceive that those who begin to love them at this age are prepared to be always with them and share all with them as long as life shall last: they will not take advantage of a boy’s green thoughtlessness to deceive him and make a mock of him by running straight off to another. Against this love of boys a law should have been enacted, [E] to prevent the sad waste of attentions paid to an object so uncertain: for who can tell where a boy will end at last, vicious or virtuous in body and soul? Good men, however, voluntarily make this law for themselves, and it is a rule which those ‘popular’ lovers ought to be forced to obey,  just as we force them, so far as we can, to refrain from loving our freeborn women. These are the persons responsible for the scandal which prompts some to say it is a shame to gratify one’s lover: such are the cases they have in view, for they observe all their reckless and wrongful doings; and surely, whatsoever is done in an orderly and lawful manner can never justly bring reproach.
Plato. (1925). Lysis, Symposium, Gorgias: English Text. (J. Henderson, Ed., W. R. M. Lamb, Trans.) (pp. 111–113). Cambridge, MA; London, England: Harvard University Press.
See this extended treatment of the issues involved. https://chs.harvard.edu/chapter/part-iii-the-symposium-sex-and-gender10-agathon-pausanias-and-diotima-in-platos-symposium-paiderastia-and-philosophia-luc-brisson/
“Marriage” didn’t make a lot of sense in this context because most assume marriage was for the creation of legitimate heirs.
Consider also the emperor Nero. Nero accidentally killed his most beloved wife while beating her and then tried to replace her. The best they could do was take a boy, castrate him and Nero married the eunuch. They remained sort of happily married until Nero was killed. Nero was thrashing around to find a way for this eunuch to bear him offspring but the technology was unavailable.
Conceptualization about many things in the ancient world was different from our own but if you want to argue 1. that same sex attraction is a common human variant (which it seems to be) AND that it isn’t terribly subject to social constructionism (which I’m increasingly doubtful about) AND that long term pair bonding between same sex individuals is a reasonable and satisfactory strategy for such individuals to pursue (they fall in love) THEN you’d be hard pressed to argue that there WEREN’T committed same sex relationship in antiquity. We should expect there to be and it shouldn’t be impossible to find them as in fact I think we find one in Plato’s Symposium.
The question remains as to why Judaism, Christianity and other major religions discouraged them. Why did THAT idea develop. Today people like to believe “because some power hungry people who wanted to control gay people’s bodies wrote it in their books” but that doesn’t really answer any of the questions. Same sex behavior was common place. Sex using slaves and children was common place. Where did we get the ideas that these things were immoral and should be suppressed?