Why is gay marriage such a scary concept to those of faith?

My pet peeve was tweeked when I was reading the news on MSN today. One of their writers wrote about how religion and gay marriage contend and of course a bunch of church goers had to raise up and spout that it's immoral and such. I want real answers not something you read in a book. Before you get mad let me point out I am a devoted Methodist and I do believe in God and Jesus.