What's so damned special about relationships? If it's about equality, then tell me, why does a childless married couple pay less in tax than a widow with a child who earns the same as the couple? I'd say the widow's relationship to the child matters to society, the couple's relationship doesn't matter to anyone but them.
Why is it legal to discriminate on the basis of marriage?
Why does any government in a secular country have anything at all to do with marriage?