Because MARRIAGE is a religious institution dating back thousands of years and is defined as a union between a man and a woman in every religion that it is practiced in.
The government should never have gotten involved in marriage in the first place (blood tests, certificates, …).
I have no problem with government establishing a ‘legal union’ law, but again, you are going to have to resolve the questions about who it applies to–most people just conveniently sweep this problem under the rug. Can I have a legal union with my dog? With my car? With multiple partners?
However, I will never, ever be okay with the government calling Adam and Steve “married”. It’s simply not the case. This says nothing about the fact that they love each other or are committed partners–I don’t dispute this is the case. They just aren’t “married”.
No lawsuit you file, PR campaign you plaster on bill boards, screaming from your bully pulpit or angry foot stomping by gay marriage supporters will persuade me to alter this viewpoint. It has nothing to do with ignorance–it’s just how I feel and frankly I don’t need to justify or explain it to anyone.