It's not so surprising. When cultures slide into decadence, they seldomly do so abruptly. It often occurs thin slice by thin slice. Once you accept one seemingly thin moral concession, it is easier to accept the next because you've shortened the distance. Not only that, but with each moral concession, you become more willing to accept still greater concessions as you become increasingly detached from reality. And we shouldn't forget the role of cognitive dissonance. The notion that the consensus about what is normal can be wrong is very uncomfortable for many.