[GOP_Party_Animal]

12) If we change God's definition of marriage to make gay marriage legal, then what's the logical argument against polygamy or even adult siblings supposed to be?

Oh, man! Just try to use this one one the "equal sign" zealots that are your Facebook friends. Get ready for a freak-out.

[MrB]

12) If we change God's definition of marriage to make gay marriage legal, then what's the logical argument against polygamy or even adult siblings supposed to be?

Any "logical argument" must be based on an assumption of the source of right and wrong. Epistemology.

Don't ask for a logical argument. Ask them to explain their foundation of knowing right and wrong, and how it's somehow better than other foundations.