2.) this is not the sole argument on any level thats why all the examples that follow always fail. we should illegally discriminate, we shouldn't deny equal rights
now on to your example
3.) incest was legal, then banned, then legal now mostly banned. THis banning is based of the danger of biological inbedding. Its banned becasue the law determined there could be a victium the child that is more likely to be born deformed.
now if doctors got this wrong, im have no issues with it
if doctors got it right and they want to fight to be married but we regulate breeding based on this that fine by me
either way, none of that has anythign to do with gays marriage
4.) not somehow, factually and equality and human rights issue.