Just picked up this book (from a used bookstore on St. Mark's, Bet. Ave A and 1st, for eight bucks). You can check it out at Amazon if you don't have similar 'luck' ;)
"A lot more is at stake in the marriage debate than the definition of a word, and this book reveals just how much. Its defense of marriage is philosophical and sociological, not theological, but people of all faiths will find it illuminating and edifying."