The reason that voters under the age of 30 are now significantly more Democratic than older voters is that they are much less likely to be married, white, and Christian. ~Alan Abramowitz 
Steve Sailer’s article  in February identified the strong correlations between marriage and children, and the affordability of both, and support for Republican candidates. This would seem to confirm that finding in a dramatic way, and it reveals much more profound long-term problems for Republicans and cultural conservatives in the coming decades. It shows that the collapse of party identification with the Republicans in my age group is the result of significant structural changes in the rising generation. Bush’s misrule and the war in Iraq are likely compounding factors, reinforcing the existing tendency of unmarried, non-Christian and nonwhite voters to support the Democrats, but probably even if these had never happened the pro-Democratic leaning of most 18-29 year olds today would be roughly what it is. The deeper problem for the GOP is that there is not really much that it can do about this. Meanwhile, this trend reveals the bankruptcy of trying to fight the culture war primarily through political means. Not only has little concrete progress been made, but while the GOP has been biding its time and using cultural conservatives to win elections the next generation has become a naturally pro-Democratic constituency for the reasons Sailer has outlined. Furthermore, even if the rising generation ends up marrying and even becoming regular church-goers in the future, voting patterns tend to be set early in life, which means that this generation is not going to come back to the GOP later. Beyond that, as marriage is generally being put off longer and longer for professional or other reasons, this pattern will keep recurring with every new cohort of 18-29 year olds. Cultural change has devoured the future of the Republican Party as it exists today. This is somewhat fitting, since it never saw fit to do much more than strike poses about culture change in any case.