In the past couple of decades the U.S. has seen a shift away from marriage, and towards cohabitation. The declining rate of marriages can be contextualized by the declining rate of religious worship given that marriage often a primarily religious act. However, marriage is also an economic act as married couples can enjoy benefits such as shared insurance benefits, shared pension/retirement benefits, shared social security, and tax benefits. These economic benefits are mostly marriage specific as common-law marriage is not very common in the U.S. This means that we can view these benefits as economic incentives to get married under the law. Given these factors, can we place the decline in marriage on lessening religious worship? This would make sense, except you can get married outside of a religious ceremony. However, the change in religiousness may point to the broader societal/cultural changes that could be contributing to a decline in marriage.
A specific change is a growth in acceptance of cohabitation a.k.a. living together with a partner outside of marriage. According to pew research data, the majority of people (in every generation) do not view cohabitation as a negative thing for society. This is a massive cultural change as it represents an implied acceptance of non-martial sexual relations, and an acceptance of living with a partner without formal title. Along with the acceptance of cohabitation, there is a trend of couples, married or not, choosing to forgo having children or having as many children.
These two changes mean some of the non-economic incentives for marriage such as children are less of a factor for increasingly larger portions of the population. While it is hard to determine the exact cause, it is pretty clear that there is a growing preference for cohabitation before (or instead of) marriage.