Yes, the Romans did have homosexual marriages in their era. This helped the State insist that marriage is what the State says it is – and the majority of Romans celebrated.
Where does the current popular support for such delusions come from?
From the rise of pleasure and companionship as the foundation of marriage, not children and commitment. Marriage is seen by most as something to be enjoyed when pleasurable, and to be abandoned when it is not. Christian failures to persuade the majority can be traced from cowardice from the pulpit and the insistence of Christians to sacrifice their children, sending them to be educated by those who hate their beliefs, in the whorehouse & drug emporium known as the public school system.
An eagerness to trust in politicians has something to do with it, as well.