I don't know of many evangelicals who want to deny gay couples their legal rights. However most of us don't want to call it marriage because we think that word has religious connotations and we're not ready to see it used in ways that offend us.
In Psalm 72 Solomon prays for power and fame but he says the purpose of influence is to speak up for others and one is the immigrant. He doesn't delineate between legal and illegal.
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I contend the state ought to do its thing and provide legal rights for all couples who want to be joined together for life. The church should bless unions that it sees fit to bless and they should be called marriages.