Protip: If you're using The Bible to justify your opposition to gay marriage, don't forget to treat all adulterers EXACTLY the same way. God makes it abundantly clear that both are equally bad. 

And if you're really being literal about your Biblical reading, please remember that you must execute anyone who has cheated on his or her spouse.

Happily, God didn't weigh in on the method of execution for this particular sin, so no need to gather stones or look up the meaning of the word "smite" in the dictionary. The method by which you murder every one of the adulterers in your life has been left to your own discretion.
Leviticus 20:10