The problem in Germany is a bit more complicated than just politics - never mind Shelter. You are right that the SPD, the Green and Die Linke made a statement. And you are right also when telling us that the government put the brake on any evolution (in the near futur) possible.
The reality is, the German constitution does NOT define marriage as a union of one man and one woman. It's only the actual interpretation of the Bundesverfassungsgericht.
And this interpretation is criticl because the marriage article is one of those that can't be altered or tossed out.
So any person with the right to appeal to the supreme court can, pratically, be sure to gain cause if they oppose marriage equality ...
That's what the jurists of the German LGBT organisation (Dt Schwulen- und Lesbenverband) told me when Germany (the state) opposed actively and passively my marriage here in France.
That sounds quite like the Irish situation, and that's why we had to have a referendum to get secure marriage rights here.
The Irish constitution protects "the family", but does not define it. The courts have interpreted it as "a man, a woman, and maybe some kids", but that's not actually what it says.
A law could have been passed to allow gay marriage, but it would have been immediately appealed.
Instead, it was decided the only viable solution was to alter the constitution, and the only way to do that is by referendum.
Since the referendum passed, our constitution now contains this line:
Marriage may be contracted in accordance with law by two persons without distinction as to their sex.
Problem solved, and not open to appeal!
B.