Since I've established that I don't think making Same Sex Marriage illegal is a logical or appropriate consequence of its being held as immoral or unbiblical by many Christians, I'd like to demonstrate how I would go about determining if it's something that we should legalize.
For me, this is about two things- a definition of marriage, and legal equality.
This probably isn't news to anyone, but people in our society define marriage in a number of different ways. Some define it as a sacred covenant between a man and a woman, or between a man and several different women. Some define it as a means for two people who are in love to get tax benefits. Some define it as a partnership and a means to co-parenting. I fundamentally view marriage as a partnership toward a common goal and a means to stability, physical, emotional, and spiritual support and companionship, fiscal security, safe sex, and co-parenting. Yes, for me, marriage is also a holy covenant. It is not, however, one that is unbreakable for any reason, (though I would absolutely acknowledge the role of commitment and fidelity in a decent marriage) and I do not think that a marriage must be a religious covenant to meet most of the goals above. To meet my definition of marriage, gender is irrelevant except for the whole co-parenting bit. To naturally conceive a child requires two genders. However, raising a child does not, and fertility does not define a marriage anyway. While having two parents together to parent children puts those children at an advantage, having children in the first place is by no means required for a good marriage. If marriage is not currently defined as I believe it should be, then yes, I would support its redefinition.
Personally, I would like to see civil marriage replaced by civil unions between any two people and separated from religious marriage. I see no reason why two siblings who have determined to spend their golden years together in lieu of attempting to marry/remarry should not have the same legal benefits as a married couple, for example.
Our legal foundation is based on the principle of liberty as long as it does not infringe on someone else's, so I think we'd need to have hard evidence to restrict the personal rights/privileges of LGBT individuals. I would think, too, that being able to marry another consenting adult who one loves is fairly basic to the pursuit of happiness. On the policy side, Same Sex Marriage benefits society by making it easier for LGBT spouses to care for one another when they age, make medical decisions, be co-custodians of their children, et c.
Here are the questions I would like to see asked and answered:
Does it provide benefits, or harms, to society in general and to specific people outside the marriage?
Does it promote healthy, stable relationships and communities, or does it intrinsically damage communities?
Unless the answers to the above questions can be proven to be "yes, it harms" and "yes, it intrinsically damages" I see no reason to restrict the abilities of homosexual couples to establish stable families. My beliefs in equality mean that I advocate for fair play, justice, and liberty, and this both for those who share my beliefs/orientation and those that do not. I believe that we should be operating from the assumption that we all have the same marriage rights until we can show that harm results from certain scenarios- I would say that polygamy generally qualifies here, as would incest, underage marriage, patriarchy, matriarchy, forced marriage, et c. In the case of Same Sex Marriage, I am not aware of any way in which allowing it would harm anyone outside the marriage.
If I were a legislator, I would look at all relevant data, perhaps commission another study or two, and try to determine what, if any, quantifiable and causal negative results spring from allowing homosexual people to get married. If I did not find any that justified continuing the prohibition, I'd support allowing it.