You have to assume first that there is right and wrong that applies to everyone. If not, then there is no debate. Each person decides what is right and wrong for his own self and disregards the "right/wrong" decisions of others.
So how do we decide this?
We must look to someone else, someone who can decide for everyone. I think we can all agree that no one is perfect. Everyone makes mistakes. So we probably shouldn't let a single person decide. How about a group of people? Thats better, but it still leaves room for mistakes. Again, no one is perfect. Then how do we decide? Have EVERYBODY decide (popular vote)? A very cliché phrase comes to mind:
"If everyone ELSE was jumping off a cliff, would you?"
Stuff like this has happened all the time in history: Slavery, smoking, carbohydrates, ect. We (or the majority) thought it was good, but it turned out not to be. Wouldn't it be great if there was someone who knew everything and wouldn't make stupid mistakes like that...
Oh hey, God.
God gave us this handy book that tells us what is right and wrong.
The Bible.
And in the Bible it is mentioned several times that homosexuality is "Detestable to God". Leviticus 18 and 20 off the top 'o my head.
Before you hit the quote button and let the flaming pour from your fingertips, remember that God doesn't hate anyone. ANYONE! He loves ALL people, no matter what they've done.
Also, real quick, "Separation of church and state" does NOT, I repeat **NOT** mean no religion in government. I means that no church (or religion) can have power over the government and vice-versa. So the Pope cannot govern or make laws. It also means that the government can't set up a state church or enforce a certain religion.