One of the problems of liberalism is they have no core beliefs or foundation. Take marriage. It's not just an agreement between two persons, male and female, or any other combination. It's a covenant involving God. Marriage is between a man and a woman AND God. Marriage without God is not marriage. It's simply a legal partnership. I have a hard time believing that God would be a partner in a marriage other than between a man and woman.
And that's the way it's been all down through the ages since God created Adam and Eve, until the progressive/socialist/liberals got involved and brought government forces to bear. Now that we have the income tax, and special allowances for married persons, and health insurance policies forced on industry by government, and other government enforced regulations that offer benefits to married persons and spouses, unions, medicare, social security, etc, all with special provisions for married persons, we NOW have a problem. Now the government wants to solve the problem they created in the first place by eliminating GOD from the marriage covenant. From the day they do this forward, it will be "Those whom GOVERNMENT has joined together let no one put asunder."
actually, they do have core beliefs...just the wrong ones.
they believe in secular humanism, hedonism, etc.
their beliefs are built on the assumption that God does not exist, or if he does exist then he is irrelevent, and downright offensive.