I've been fielding a lot of questions on the proposed marriage amendment in Indiana that would essentially define marriage as being between a man and a woman.
Some ask: "there are other laws that allow violations of Church teaching, and yet the Church doesn't fight those; why should we fight against homosexual marriage?"
The answer here is real simple - the Church says there are certain things that governments can decide for themselves, but there are other issues that, if the government of a society gets them wrong (goes against the Church and Christ), then the society will fall.
So...a Catholic is COMMANDED to fight against certain violations of Church law - and the redefinition of marriage is one of those issues.
So it isn't a state law, as far as I'm aware, that cheating on spelling tests is illegal. The Church doesn't COMMAND Catholics to fight for laws that punish spelling test cheaters, even though cheating on a spelling test is morally wrong.
However, homosexual marriage IS something we are commanded, by the Church, to fight against in the civil realm.
You might not like to hear it spelled out this way...but a Catholic doesn't have the option as to whether or not they are going to care about legislation concerning the redefinition of marriage.
People looking for where, exactly, I'm getting this teaching will want to turn to paragraph 229 of the Compendium of Catholic Social Teaching, where we read:
"Although legislation may sometimes tolerate morally unacceptable behavior, it must never weaken the recognition of indissoluble monogamous marriage as the only authentic form of the family"
The first part of this teaching is derived from John Paul II's encyclical Evangelium Vitae, 71.
In that letter, Pope JP II was using St. Thomas Aquinas's Summa I-II q. 96 a. 2, which answers the question "whether it belongs to human law to repress all vices" - the answer being "no"