Kieran was right. It doesn't matter how many times you answer the question, you ignore it and ask the same question over again. I'll answer it one last time.
Through legalization of SSM, your are inferring that a homosexual relationship is on the same footing as a heterosexual relationship. Most people don't hold this to be a core belief.
The majority of Americans realize that homosexuality, even though it is a deviant behavior, hurts no one. Therefore, the regulations surrounding it are being universally lifted in spite of the "yuck" factor.
When you put homosexuality on equal footing as heterosexuality, you are no longer teaching tolerance. You are forcing the community to accept a deviant behavior as normal. Again, this goes against some people's core beliefs.