Of course number of people believing something does not make something justifiable. Think about slaves!
Also, please do not think that people oppose gay marriage based on nothing.
I don't see any problems two humans getting married and living together. However, I have problems with marriage benefits. As APA rejected most of pro-gay research because they are biased and small sample sized, gay marriage is yet proven to be positive to our society. Why we give out tax cuts to heterosexual couple? It brings positive effect on society, and thus government encourage heterosexual couple with benefits. The burden of proof is on homosexual people that indeed they bring positive effect.
It is like trying to get States to recognize my bacon eating. If I can prove that me eating bacon brings positive effect, state should provide me benefit for my bacon eating. And of course, state cannot ban me for eating bacon.