The Bible as read by Christians, mostly. If you know of some other argument please let us know - I keep seeing allusions to something other than the Christian version of the bible, but never see the actual claim.
So the Muslims, Jews, Buddhists, Hindus that you referenced earlier; those societies all wanted homosexual marriage for hundreds or even thousands of years, but were kept from doing so by Christians making Biblically-based arguments? How long before the birth of Christianity, or before the collation of the biblical books was this occurring, would you say? Can you tell us anything about the nature of the Christians' time travel technology?
Sorry fistful - you missed the point. And are now obviouly making the error of moral argument by dictionary. All of them have widely different versions of what a marriage entails. If the definition of "marriage" is so obvious or universal, that isn't possible. The translator who came up with the equivalent terms doesn't get to determine at all what are the necessary and sufficient conditions of a marriage - that is a judgment call, not a definition.
Buddhists, muslims, and Hindus and various non-Christian societies all tolerated or formally recognised same sex arrangements over the years. The fact that Christians didn't translate those words as "marriage" into English is a judgment call, which you're now trying to twist into a moral argument because it comes from the dictionary. Well, it ended up there because of a judgment call in the first place.
So I'll ask again - what's this non-religious basis for discriminating against gay relationships? If it's just that other socieities do it, I shouldn't have to explain why that's absurd.