The thinking is that until now gay marriage hasn't been seen as a partisan issue and several of the more liberal republicans have come over on the side of supporting gay marriage. With Obama's latest announcements this is going to be harder as they will now seem more left leaning than before Obama came out for this cause.
Also it is debatable if we want an election where gay marriage is again an important issue. Some right leaning evangelicals who would otherwise have been skeptical about voting for a Mormon might now feel more compelled to do so.
I still don't understand why anybody but gay people would feel that it is so important but that seems to be the world we live in. Myself I feel strongly about this issue because if you replace gay with for instance black it just becomes so obvious that this is discrimination against a minority (Remember it wasn't that long ago that interracial marriages were illegal too in many states).