Why Hillary (and all Dems) should support Gay Marriage

Yesterday B Man praised Senator Hillary Clinton for her "excellent" framing of the issue of gay civil unions. Many of the comments in that diary echoed that praise. In response to noting that, in my opinion, Senator Clinton should support gay marriage, and nothing short of gay marriage, a few commenters asked what the legal difference between civil unions and marriage are, implying I should be content with promoting civil unions for the time being, as they're more easy to achieve and are legally the same thing. Only they're not legally the same thing.