The media prefers to focus on lack of unanimity in the Republican party on marriage, but the truth of the matter is that there are pro-marriage Democrats as well:
One third of House Democrats broke with their leaders today and kept their names off a brief urging a federal appeals court to strike down the Defense of Marriage Act, which limits federal recognition of marriages to those between one man and one woman.
The majority of Democrats joined House Minority Leader Nancy Pelosi's argument, which her office described in a news release as "the Democratic Members' brief," but the break among Democrats offers a glimpse at remaining divisions inside the party.
... Although some Republicans had been asked to join the earlier brief, Pelosi's spokesman, Drew Hammill, told BuzzFeed that none chose to join that filing. Of today's filing, Hammill added, "We had received no indications then or since that any Republican Member were willing to challenge their leadership's decision to expend taxpayer funds on defending DOMA." -- BuzzFeed Politics