We all know that there are still people in America that don't want gay marriage to be legal.
If people are opposing gay marriage just because the Bible says that it's wrong, how come they aren't trying to shut down Red Lobster for serving shellfish (Leviticus says shellfish is unclean) or trying to stop retailers from selling clothes made from more than one type of fabric?
The Mosaic Law was repealed in Paul's epistles with the exception of the moral law which Paul affirms. As TJ noted, this issue was settled within the apostles' lifetime.
And how does two guys or two gals being married even affect other people's marriages?
Your problem here is that you are taking a secular moral framework and asking Christianity to justify itself according to the framework. Christianity rejects the framework altogether.