Could this be a response to the backlash DC faced after appointing anti-gay marriage crusader Orson Scott Card to their Adventures Of Superman anthology? In Batwoman #17, by J.H. Williams III, W. Haden Blackman and Dave Stewart, the Medusa storyline concludes with Batwoman, Wonder Woman and Hawkfire (Bette Kane) victorious, and Kate Kane revealing her identity to Maggie Sawyer. We don't get to see the Gotham City police captain's reaction though, as Batwoman says, "Marry me, Mags," and kisses her.
This wouldn't be the first same-sex marriage (provided the nuptials even occur of course) in comics, and Marvel have DC beat by quite a margin with their X-men/Northstar wedding. However, the timing is certainly coincidental. Williams doesn't address the proposal directly, but he does hit that something big was on the cards for the character in a post from his blog, via CBR.
"...After weeks of planning and plotting, figuring out just how those pieces moved and fit into a bigger construct, we knew where we were going with her, where the bigger fantastical story interacted with more personal aspects of Kate Kane’s life. So the current few issues of the tale have all been set up since the beginning, since Batwoman’s first Issue 0 (yes, there has been more than one). And it’s all culminating now, into issue 17 that is out now. It’s an important issue in many respects, not just to Kate’s story, but to ours as well."