They feel guilty and want to stamp out their sin by removing this from all religions.
They think when the power of the state forces everyone to accept them, they will finally not feel guilty anymore.
Very good point. Liberals often confuse, perhaps deliberately so, cause and effect. They think if they can force someone at the point a gun to say what they want to hear that their views will be sanctioned. At its core, modern day "liberalism" is the worship of the null.