Liberal Christianity has to attempt to broaden it's support base to survive (and will do that by making political decisions, like this one) but that base of support it's trying to reach out to are precisely that group of people which is becoming far less religious and far more secular. In doing so, it alienates those which are arguably most committed to the doctrine of the faith it proclaims, sending them into the arms of the conservative elements of that faith. These kinds of actions might buy it time in the short run, but will ultimately only further the disintegration of the liberal churches, and in fact may end up accelerating it.
http://publicreligion.org/newsroom/2013/07/news-release-1-in-5-americans-are-religious-progressives/
They'll grow out of it eventually. What's the point in being religious if you aren't going to accept the moral dictates of the religion you claim to adhere to?