It seems to me the different marriage customs of the world are all manifestations of an underlying, gene-based instinct or propensity in humans to pair bond. There seems to be no other plausible explanation for the ubiquity of marriage.
Of course, pair-bonding does not preclude polygamy. Polygamy is merely a condition in which one of the partners has formed multiple pair-bonds. e.g. J0nes has formed a pair-bond with Smith and Stewart each.
One thing pair-bonding does not imply is group marriage. That is, marriages of more than two people in which every partner is equally married to every other partner. Humans now and then try such arrangements. Yet, in every documented case of group marriage, the participants reverted to pair bonds within six years.
This instinct to pair bond also seems to be present in both heterosexuals and homosexuals.
So I don’t really understand the notion that marriage is traditionally an exclusively heterosexual institution established by some deity.
On the contrary, it seems pretty clear that marriage is ultimately established by our genes — unless you want to add another layer of explanation and claim our genes were established by some deity.
Furthermore, it seems equally clear that both most heterosexuals and most homosexuals have some desire to pair-bond. So, to argue that marriage is an exclusively heterosexual institution would seem to be factually incorrect.
Now, I would not argue that anyone has a right to pair bond simply and only because it’s in their genes to pair bond. It’s in most people’s genes to go to war, but that by itself doesn’t mean anyone has a right to go to war.
But I would argue that the notion pair-bonding is an exclusively heterosexual tradition is evidently false. And I would also argue that — since pair bonding is the biological basis of marriage — it would seem to be very difficult to argue on the basis of its origins that marriage ought to be limited to heterosexuals.