The argument is that marriage is between a man and a woman. Altering it's definition to include a behavior which goes against nature itself, though legal, is not just.
Black persons cannot control or change their skin color, the comparison is self serving. The practice of homosexuality is a sexual behavior, a behavior, which a person can CONTROL and change or choose not to control or change.
What are our laws based on ? Judeo Christian values, put that aside. The unity of a man and a woman is recognized by nature, as men and women are physicallymade up and their ability to pro-create, as a natural union, anything else is a perversion or mutation.