Of course, same-sex marriage started out as a conservative idea, and lefties resisted. That was back in the day when marriage was patriarchy and oppression.
Here's a prediction: Once gay marriage becomes the norm, the left can get back to critiquing marriage. That topic got backburnered.
ADDED: Recommendation: Conservatives should get on the side of same-sex marriage now, make it part of what is traditional, and embrace the "marriage" ideal generally (and for all) going forward. The left will go off in another direction, and good old marriage will be yours once again.
AND: If you don't remember when lefties called it right-wing to support gay marriage and gays in the military, read "Richard Goldstein’s Heresy Hunt," and article from 2001, referring to a Village Voice writer who despised that notorious right-winger, Andrew Sullivan::
... Goldstein decried the so-called "gay right" because he says that they bring the issues of gay marriage and gays in the military to the forefront, making them more prominent, while they should be arguing for an end to workplace, housing and public accommodation discrimination. "They believe in civil equality, not equal opportunity," he said....And watch "The Great Gay Debate" from 2002, with Andrew Sullivan, attacked from the left, at his fiery best. Conservatives missed a great opportunity a decade ago.
... Goldstein said we should revile the gay right because they are "a masculinist group of gay writers." They are men and women who worship and aspire to traditional masculinity and "cannot see beyond their privilege." He then equated masculinism with marriage-and-military advocates: "In times of war, masculinist values come to the forefront and feminist values recede.... If these people prevail.... the masculinist version of homosexuality will come to dominate the movement. ... It is the most dangerous thing we face today, I believe."