Assuming homosexual marriage is sinful and assuming homosexual marriage when it occurs would occur in society, permitting homosexual marriage would be propagating sin in society.
As I understood it, it is homosexual
behaviour that you find to be 'sinful' according to the bible? Whether or not marriage is legalised for gay couples, they will still participate in this 'sinful' behaviour. Homosexuality is already becoming accepted and denying them the opportunity to marry won't change this 'propagating sin in society'. What it does mean is that two people who love each other can't declare a lasting commitment to each other. To go back to the golden rule, or simple empathy to give it another name, how would you feel if you were not allowed to marry the person you love?
Voting against gay marriage has no effect on the sin, but it does punish the sinner.