Being gay is by definition against nature. Think about it, if the vast majority of society decided 300 years ago to embrace homosexuality, where would humanity be today?
An argument that homosexuality is commonplace in the animal kingdom is 1) false and 2) links homosexuals with animals which is offensive to many. This analogy highlights how poor the pro gay marriage argument is.
Homosexuality was NEVER broadly embraced by the most advanced civilizations. I am amazed at the sweeping statements made with no historical support. Homosexuality was embraced by the elites of Greece but not writ large in its society. No advanced Asian civilization embraced homosexuality. No Indian, African or Middle Eastern advanced civilization broadly embraced homosexuality. Neither the Romans nor Brits broadly embrace homosexuality. Did certain elites, certainly, but no advanced civilization broadly embraced it.
Ignorance re Catholic history is shocking. The Catholic Church established the first universities. Do you realize the importance of the Catholic Church on the development of science? I will site but two priests that change history and there are literally hundreds over 2000 years. Copernicus discovered that the earth circulated around the sun not the opposite. Mendel is the father of genetics. To state that the Catholic Church is anti-science is either to display profound ignorance or to lie. No religion or even any University can claim to have contributed more to math or science than the Catholic Church. Fact. I encourage people to read JP II's Faith and Reason.
If you don't like the 2000 year span then look to human society. Traditional marriage has been broadly embraced by human society for 5000 years. Have the Japanese been misled by the Catholic Church, too?
On my side of the argument I have essentially recorded human history (broadly speaking) and the vast majority of countries in the world today in support of traditional marriage. The other side has the recent and current support of elites and some opinion polls. I will let the readers decide the better argument. Again, I respect all opinions and would that others would as well.