Christians Only: Why do you believe there shouldn't be same sex marriage?

I want a clean debate here. I’m a Christian and I know we don’t have the right to oppress, judge, or order someone what to do. Why don’t we just live our lives and lead by example instead of trying to control everything around us?
Why do we, for the most part oppress same sex couples?
Why are you against same sex marriages?
Why are you for same sex marriages :) ?