"“I wanted to ask you a couple of questions while I had you here. I’m interested in selling my youngest daughter into slavery as sanctioned in Exodus 21:7. She’s a Georgetown sophomore, speaks fluent Italian, always cleared the table when it was her turn. What would a good price for her be? While thinking about that, can I ask another? My Chief of Staff Leo McGarry insists on working on the Sabbath. Exodus 35:2 clearly says he should be put to death. Am I morally obligated to kill him myself, or is it okay to call the police? Here’s one that’s really important ‘cause we’ve got a lot of sports fans in this town: Touching the skin of a dead pig makes one unclean. Leviticus 11:7. If they promise to wear gloves, can the Washington Redskins still play football? Can Notre Dame? Can West Point? Does the whole town really have to be together to stone my brother John for planting different crops side by side? Can I burn my mother in a small family gathering for wearing garments made from two different threads? Think about those questions, would you? ”
— Jed Bartlet, The West Wing
We don't follow everything that the bible says because much of it is outdated. Examples above. Deciding to say that gay people can't get married due to some references in the bible about homosexuality being a sin is flat out discrimination. Imagine saying that only white people can get married, though black people can have civil ceremonies. It's ridiculous, and there's not really any way of sugar coating that.
Sure, the majority might not care, but it's still something that's representing them as less, or wrong compared to heterosexual people. "No, you can't get married as you are built wrong in a genetic fashion". Equality shouldn't be done in half measures.
Additionally, saying that gay marriage is wrong because it won't lead to the procreation of children; does this mean we should also strip the right to get married from people who are infertile and physically can't have children? Can we tell people who don't want to have children because of whatever reason they can't get married because they'll be missing the point?
As someone with gay friends who do support gay marriage, it's something I feel pretty strongly on haha.