I think your right about germany, never the less, if our scotus did it, there would be nothing that would at that point stop say the president, from essentilly ignoring there ruling, and saying "go ahead and endorce it yourselfs" (this actually has happened in the US, I think it may have been Jackson, and I think he actually used those words).
Never the less, Declaring an amendment unconstitional, would essentially draw a crisis on the spot, as the system of checks and balances would essentially be destroyed. The check on the supreme court is constitutional amendments, there is no other real check. Appointments aren't even a real check, and impeachments are not a solid check either, those amendments are what keeps the court from essentially usurping all the power of the executive and legislative branches.
YOu could and would have an instant dictatorship if they could declare amendments unconstitutional, hence why they dance around it by calling the document living and breathing. More likely then not, if say all 9 said an amendment was unconstitional, then there would be nothing to stop congress and the president from also ignoring the constitional in total either, and basically ignoring SCOTUS, or simply declaring they they can now outrank scotus. All crazy and hypothetical to be sure, note, that the constitution is explicit enough to declare that it is the supreme law of the land, and specifies how to make an amendment, and that it does trump anything and everything else. It also specifies the only way to change an amendment or get rid of one, it to amend again. SCOTUS overruling the law of the land, is hence illegal, by violating the check and balance, and usurping the amending process would bring about more then impeachment, it could actually result in there arrest. (scary stuff).
Marriage is not one of those.