"The exclusion of same-sex couples from the benefits and responsibilities of marriage, accordingly, is not a small and tangential inconvenience resulting from a few surviving relics of societal prejudice destined to evaporate like the morning dew. It represents a harsh if oblique statement by the law that same-sex couples are outsiders, and that their need for affirmation and protection of their intimate relations as human beings is somehow less than that of heterosexual couples."