Because some people in government are now hostile to marriage as God defined it, in order to protect godly marriage we must remove the power of the State to define and regulate marriage. Then anyone will be perfectly free to enter into whatever relationship they want. However, they will not be able to call upon the police power of the state to force me to recognize a “marriage” that I don’t want to accept.
If we remove government from marriage and return it to the private sphere where it came from, lesbians can still marry each other, but they won’t be able to use government to force me to sell them wedding cake or artfully photograph their “marriage”. They will be free to do as they please without being able to coerce anyone else about their private affair.
But this was never really about marriage. For homosexuals it was social engineering and payback. They could bust marriage as God defined it, they could rub Christianity’s nose in it, they could punish people who objected and they could force anyone who was too vocal to shut up. But what they really want is approval, and it just galls them that a lot of people will never give it to them.
Marriage is recognized by the state as an important social construct, promoting lifetime unions of male-female so that children that might be conceived will have both their mothers and fathers to raise them. This helps children by giving them both a mom and dad, and it helps society by encouraging parents to take joint responsibility for the children that they conceive, reducing the chance that the children will become charges of the government.
What the law needs to do is strengthen marriage to promote this noble purpose: reinstate fault-based divorce laws, ditch same-sex “marriage,” and reduce single-mother benefits.
You can already do that, so why don't you?