I don't have the anthropological data on hand, but in societies that where same-sex marriage existed, such as the Dakota people, it usually involved a man having multiple wives, and at least one of the wives is a biologically male person living as a female. Such wives were regarded as valuable because they can do household chores that involve strength. It was deemed necessary to have a biologically female wife, though.
For female marriage, I've heard about one society in Africa where for some reason involving inheritance, if an older woman has no offspring, she can live as a man and take a younger female wife, who will have sex with another man. The children of this younger woman were legally the children of the older children.
In summary, all the ancient same-sex marriages were same-sex, opposite-gender, involving a transgender person. If the idea of transgenderedness gets accepted by Romans, then same-sex marriage is a possibility.