This may not be anything to get particularly worked up about. Although the trend may lead to delayed childbirth, there’s nothing intrinsically wrong with waiting until your 30s to get started on a family. People with a few more years as a single may have more time to find that special someone.
On the other hand, the decline of marriage may contribute to the existing trend of professional women waiting later and later (in some cases too late) to have children. The increasing cost of childcare makes single-motherhood almost completely untenable for women in some advanced professions (a young postdoc’s salary will barely cover daycare in some places). Of course many cohabiting couples are models of fidelity notwithstanding the lack of vows, and a responsible father will contribute his fair share to the family. However, the formal arrangement probably brings some sense of security and commitment with it.
Of course there are factions that will react with utter terror and alarm to this situation, namely the family values pundits. The trend toward increasing cohabitation (living in sin!) is taken as a sign of moral decline; this will be added to that weight. They are likely to be most affected in this regard by anecdotes of people choosing cohabitation as a way to avoid the messiness of divorce. Probably divorce is a contributing cause to the current trend; witnessing their parent’s travails may have put some of the twentysomething population off the idea of marriage. The irony here is that the “Focus on the Family” crowd have been frothing after the wrong culprit for so long. The real danger to the institution of marriage isn’t gays who want it; it’s straight people who don’t.