I'm not sure Germany's image (and self-image) as a sensible country is entirely true. What is true is that German bureaucracy inhibits the country from going mad most of the time, but when Germany does go mad it can take a long time to correct course.
Also, it's difficult to overstate how obsessed German politicos are with American politics, and almost invariably that means blue state American politics.
In terms of the politics of the thing, the Greens and FDP wanted this; the FDP, in the face of collapsing support, are under internal pressure to leave the coalition; the SPD's leadership deferred to their LGBT caucus who told them it was really popular with the kids; and since the Bundestag vote shaped up mostly along left-right lines, that kept the centre-left on side.
It's been one of the issues behind the split in the Linkspartei, but more a symptom than a cause. The key factor there is that the Berlin chapter has an outsize influence in the federal party, and the Berlin chapter is dominated by a bunch of hipsters in Mitte and Neukölln who are indistinguishable from the Greens. (Think Camden and Islington's influence in UK Labour.)