JBroll
I MIX WITH PHYSICS!!!!
This is a really hard question to answer because I didn't invent marriage. I would contend that God created marriage so He gets to set the rules for what's right and what's not. Biblically I can show the immorality of homosexuality, I can also show you practically the reasons for hetrosexual marriage/relationships. However beyond there I can't contend for much against it, which is where usually a person coming from a non-Christian background would find more authority. I also have to be careful because there is still a distinction between marriage and relationships.
There's plenty of polygamy in the Bible, where's my harem? I want to see televangelists 'pimpin' it like King David, bastards...
Marriage outdates your religion and its predecessors, and has a quite unflattering background as a property exchange ritual, so for any religion to claim 'ownership' of marriage is simply absurd.
Homosexuality occurs in nature, and isn't something people have control over. Any condemnation of homosexuality can't be taken more seriously than condemnation of shellfish, proximity to menstruating women, or backtalking children.
Jeff