Actually your take on marriage is demonstrably wrong in historical terms: Roman civil marriage was also monogamous and owed nothing to Jewish Scripture. It was Roman civil marriage, not Jewish marriage, which became the basis for Christian marriage. Marriages between Christians were Roman civil marriages undertaken with the blessing of the bishop until at least the time of Justinian (whose novellas permit either priests or notaries to conclude marriages), and were not a strictly religious affair until the transfer of all marriages between free citizens to the Church by Leo VI in the 9th century, and all marriages by Alexis I in the late 11th century.
How the transition from civil to Church marriage was handled in the lands which ceased to be under Imperial control thanks to barbarian invasions, I don not know: perhaps one of our knowledgable Latins can fill in.
Marriage is a natural institution, Church (or other religious) marriage is the sanctification of that institution, and civil marriage its acknowledgment by the state. The baleful effects of 'gay marriage' in our already debauched culture lie primarily in the redefinition of the natural institution as the content of its state recognition so that the natural institution is abolished and replaced with a statist construct.
Christianity came from Judaism, which was long before the Romans were even a ‘shot in the dark.’ Yeshua was a Jew and there is a reference to marriage in the Gospel (forgive me, I'm not a Christian and cannot recall the text).
Now, you may make the argument that the basis for Roman Catholic marriage (which some Christians would argue is of a pagan nature) is from the Roman pagan culture.
Adultery was certainly condoned and sanctioned for the male in Rome and in Greece, something the Judaic culture had forbid, despite their history of polygyny.
But, even so, the nature of marriage is biological, establishing who the parents were for inheritance. It is a creationist model nevertheless, as even the pagan Romans, pagan Greeks and other ancient pagan cultures were. It just so happens that marriage in this country is based on the Judaic model, otherwise we would not have adultery as legal grounds for a woman seeking a divorce.