It'll happen eventually, probably sooner rather than later, even with Republican controlled houses. Equal treatment for gay Americans was a wedge issue the right has used to attack the left forever. Within the space of a decade it's getting to the point where it's no longer effective to weaponize like that.
The thing a lot on the right don't realize is that's the issue is continuing on its trajectory and is on it's way to becoming a wedge issue that the left can attack the right with to great effect, rather than just staying a non-issue. People are becoming less bigoted and less tolerant of bigots. We've already go several protected classes. It isn't a hard argument to make and win the way public sentiment is going.