There were many bulwarks before gay marriage in America. One was in the when the Bible was removed from the schools in favor of moral-less atheism. Another was when no-fault divorce was legalized showing that marriage vows are not really vows at all. And there were more before these.
The USA was a Christian nation from it's founding and all of its morals and principles are based on Christian thought. With each bulwark that was crossed, America moved further and further away from its roots which gave validity and reasoning to its morals and laws.
Might be more of a young adult series, but the Leviathan series by Scott Westerfeld is pretty good. It's set during WW1. The Germans have developed steampunk tech while the British have developed hybrid animal bio-tech. The first book follows them on a mission to deliver a package.