In 1992, 78% said "Yes." In 2012, 42% said, "Yes." The results were the same for men and women.
There are several factors behind the results.
A change in life priorities.
These trends show that men are somewhat more focused on career and less on family. Men also do not see themselves as the sole breadwinner anymore. They expect their spouses to be engaged in careers full time.
Longer work week.
The average number of expected hours per week has gone from 58 hours per week in 1992 to 72 in 2012. That's 14 more hours per week. With working that many hours, many young adults don't see how they can be a parent.
Student debt.
Many young adults are leaving college with a lot of debt which constrains them from the prospect of having children. They want to have children but don't see how they can in light of all the pressure of paying off debt.
Economic uncertainty.
Millennials were in their formative years when 9-11 occurred. This had a profound impact on them. They also entered college as the Great Recession hit.