The recent huge demonstration in Paris against gay marriage had the right idea. It would be unpersuasive for them to appeal to the authority of the Bible, since huge numbers of French people don't take Scripture seriously (except for the Muslims). Instead they focused in on the fact that children flourish best in a family with their own married father and mother. In a "gay" family any children will be, by intent, by pla, --- by design --- bereft of one or the other. It's the deliberate creation of brokenness.
Not just Catholics, not just Christians, not just “believers,” but any person who cares to think about it.
You are right. But, scripture also says that God gave man the knowledge of right and wrong. People DO know the difference in their hearts; it’s their heads that are the problem. You are correct about any reasonable person could figure it out. The mere fact that the sex act between two men is absolutely unnatural in of itself is proof that it is wrong - the design of male and female proves it.