Why Do Unmarried Couples Who Expect to Have, or Already

Why do unmarried couples who expect to have, or already do have, a child outside of marriage not marry as frequently as these individuals who belonged to older cohorts?

A) social norms have changed regarding out-of-wedlock childbearing
B) social norms have not changed regarding marriage
C) out-of-wedlock childbearing simply doesn't occur as much as it used to
D) more educated women are most likely to find themselves having a child outside of marriage and they are also most likely to be economically independent