To be clear, I don't have a strong opinion about this, and I'm not making a claim that marriage is a good or a bad thing, I'm just answering your question.

The usual argument that says that the government disincentivized marriage is through social safety nets like welfare, particularly ones that pay more for single parents without a spouse in the house, as well as mechanisms like child support payments.

The usual argument that says society has disincentivized marriage is basically through access to sex. There used to be a culture of enforced monogamy, where there was enormous social pressure to be married before having sex, and especially before living with one another. Now, that stigma is largely gone, so the incentive to get married has diminished substantially.

Again, not making a value judgement, just answering the question.