Lisa Miller writes on gay marriage in Newsweek:
First, while the Bible and Jesus say many important things about love and family, neither explicitly defines marriage as between one man and one woman. And second, as the examples above illustrate, no sensible modern person wants marriage—theirs or anyone else's —to look in its particulars anything like what the Bible describes. "Marriage" in America refers to two separate things, a religious institution and a civil one, though it is most often enacted as a messy conflation of the two. As a civil institution, marriage offers practical benefits to both partners: contractual rights having to do with taxes; insurance; the care and custody of children; visitation rights; and inheritance. As a religious institution, marriage offers something else: a commitment of both partners before God to love, honor and cherish each other—in sickness and in health, for richer and poorer—in accordance with God's will. In a religious marriage, two people promise to take care of each other, profoundly, the way they believe God cares for them. Biblical literalists will disagree, but the Bible is a living document, powerful for more than 2,000 years because its truths speak to us even as we change through history. In that light, Scripture gives us no good reason why gays and lesbians should not be (civilly and religiously) married—and a number of excellent reasons why they should.
This is a very useful statement that will reach a wider public.
Update: I have posted a longer response to Lisa Miller's article on the Episcopal Cafe website making the case that much more than "mutual joy" is conferred in the sacramental benefits of marriage.