If you look up family demography, you'll see a lot of research on the rise of cohabitation and delaying marriage, which is why it appears as though the marriage rates have declined.
Delaying marriage is typically attributed to longer time spent in education (and a bit on selfishness, like George points out), while cohabitation is typically attributed to the decrease in religiosity, as well as in "trying out" a partner before marriage. It's also a prefered option for those who have been married, and don't want to go through all the trials and tribulations of marriage again.
I don´t know how the exact numbers are. But in a modern society you will find different understandings by women in self conception, financial and gender role. So I think it was to expect, that many young males and females refuse marriage as normal traditional behaviour, especially if they don´t aim for reproduction by own children.