The United States is NOT a Christian nation.
It is a nation with a lot of Christians in it.
If the state is to allow an adult to declare their commitment to another adult and grant certain rights in property and entitlement benefits with that declaration, then any restriction of that right which refuses to recognize any declaration by any set of consenting adults is discrimination on its face just as surely as interracial marriage bans used to be discriminatory.
If you happen to be offended by that concept, so what? You have no right against being offended. The rest of the population need not lower itself to your standards because you're such a sensitive soul so easily made queasy by two men kissing? Grow a pair, huh?