Because that's all it is. Gay marriage doesn't hurt them in any way, it doesn't hurt anyone in any way, they have no self-preserving interest in this, they simply want to deny other couples the right to get married.
What really pisses me off is they have the audacity to call themselves pro-family when they're the exact opposite. They're campaigning AGAINST families, they want to make families illegal, they want to break up families. How can they call themselves pro-family?
I suppose they are working on the unbelievably arrogant basis that a straight couple is a better family than a gay couple. This is complete crap. A family is a family, whether they're made up of Mum, Dad and their two children or two Mums and their children or Mum and her children or Dad, his children, his parents and great aunt Frieda. How dare they say that one family unit is automatically better than another?