It's not gay marriage, as just MARRIAGE altogether. DC is anti-marriage for any of its heroes, right now.
They're pretty much against anything that threatens the "youthful" vibe they're going for with their heroes.
Superman's marriage was erased, and he quit his job.
Flash's marriage was erased.
Aquaman got to keep his wife, but abdicated his throne.
Arsenal's daughter was erased from continuity.
Batman's son was killed off (I realize this was planned from the beginning, but still...)
Green Arrow is too young for Conner Hawke to exist, Alan Scott is too young for Jade and Obsidian to exist.
Amanda Waller was changed from a middle aged, plus sized mother who was inspired by the children she lost to crime into a sexy young secret agent.
I'm sure there were more, but these are the ones I can think of off hand. Any signs of maturity or adulthood are erased in the name of youthfulness.
Marvel's guilty of it as well. Jean Grey was killed so Cyclops could start sleeping with Emma Frost. Spider-Man sold his marriage to the devil. The only superheroes who are allowed to stay married are Reed Richards and Sue Storm.
I don't think it has anything to do with the "gay" part of gay marriage. If that was the case, we wouldn't have Batwoman, the new Alan Scott, Bunker, or Midnighter and Apollo in the new DC Universe. I think it has much more to do with the "marriage" part, because heroes aren't allowed to settle down.