The Catholic Church condemns moral relativism and says the truths of morality are objective truths. Even Catholics who belong in the Dark Ages and who are fundamentalists make mistakes in being consistent with this condemnation. You hear their spokesmen saying they think same-sex marriage is a sin or they think its bad for the spiritual health of society. This denies that the doctrine that same sex marriage is nonsense and a sin is objective truth. To say you think something is wrong is to succumb to moral relativism which is simply a fancy name for the doctrine that morality is just opinion or what people think and is therefore not to be taken seriously. It is true that in some cases it is only our opinion that a certain course of action is the moral one or the most moral one but not in all. And the fact that we need to have opinions at times about what is moral or not does not prove that morality is just about opinion.
To agree with the Church, the Catholic says they know same-sex marriage is wrong and they know it is unhealthy for the souls of the people. Objective morality presupposes that you can know what is right and wrong.  To, "But x being true does not mean you have any way to know?", they say God teaches them. 

They may say people have a right to disagree with them. That is denying that the truths are objective truths. You cannot argue with facts. And if people have a right to dispute what is objectively true, then you must encourage them to disagree as much as you would encourage them to agree. Thus you are only wasting your time speaking at all. Also, even if you don't urge the legal system to punish all objective evils, you must consider it obligated to admit the existence of objective good and objective evil. It must make it illegal for the state to hold that morality is mere opinion.
Any religion claiming that same-sex marriage is objectively immoral is expressing a willingness to force its view on people.

No Copyright