I don't care about your relationship, I don't want to know about your relationship, and I don't want the state, which represents me, endorsing it.
It enshrines a redefinition of marriage in the law that makes marriage purely adult centric, separating it from the interest of begetting children and preserving those childrens' ties--- so far as possible --- with their natural fathers and mothers, their siblings, and their whole extended families.
It would be taught in the schools and make it discriminatory to highlight and promote real (man-woman) marriage. Same-sex "marriage" also requires denying or minimizing the significant benefit which every child enjoys of having a father and mother who are married to each other.
Same-sex "marriage" also threatens religious liberty and isolates citizens, churches and organizations that refuse to cooperate with it as a matter of conscience.