The problem is that marriage represents 2 things in our society.
A religious ritual (which existed long before the birth of Christ, and so was defined long before any biblical reference to it. This is variant between religions, and up to the leaders of each religion to define the context in which they teach it. Those unhappy with how it's taught are free to go to another religion that practices what they believe, just like all other aspects of faith.
A state institution which protects and guarantees rights between those persons who have made an agreement under the laws of that state. In that regard, those rights and protections should not be isolated to any one religious faction, as we are a nation which allows all people to practice that which they choose with equal coverage under the law. Because of this, a persons sexuality, race, etc should not be allowed to be a factor... as this is about equal rights, not religion.
I think the failing here is when any specific set of religion tries to enforce it's particular brand of morality on the rest of society, generally through the force of manipulating government.
Christians like yourself have seemingly claimed the right to define marriage based on a very limited set of teachings from their chosen holy book, when even in their own bible if you read deeper it was ONCE defined as something MUCH different. Not something gone into willingly by both parties based on love, but based on financial arrangements made by the parents of those to be wed. Christian kings married their daughters off for political power. Their sons married into lands and wealth. Is this your definition of morality? Is this the fine christian way of things?
2 Corinthians 6:14 says:
"Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness?"
Should there then be laws put into place to prevent an atheist from being married to a christian, regardless of sexuality? How far would you go in seeking a christian version of sharia law in regards to the institution of marriage?
Religion is much like a penis... best when not shoved down other people's throats against their will (though if you do, based on Deuteronomy you're obligated buy them from their father and to marry them even if they don't consent).