The German Bundestag voted today to legalize same-sex marriage (& adoption). Chancellor Angela Merkel allowed the conscience vote at the last moment to avoid it being an issue in elections this fall, but she herself voted against it because apparently she doesn’t feel it’s constitutional? 
What exactly does Germany’s Basic Law say about marriage and is Frau Merkel correct in her interpretation? Is this going to be challenged before Federal Constitutional Court? Or was the vote in the Bundestag just the first step in amending the Basic Law?
Marriage as referenced in the constitution (Article 6), but not defined.
The majority opinion of the Constitutional Court, if I understand the 1993 ruling correctly, is that in 1949 when the Constitution was drafted it was universally understood that marriage = man + woman. It is very probable in my opinion that the Constitutional Court rules that a constitutional amendment is necessary.
When registered partnership for homosexual couples was introduced in 2001, there was an intense debate on it. An opinion which was strong at the time held that there was a constitutional requirement for differences between marriage and homosexual registered partnership (Abstandsgebot), i.e., a constitutional principle that with respect to the legal consequences arising from them, there must still be a difference between the two - registered partnership can be largely, but not fully, modelled upon marriage. This view was rejected by the Constitutional Court, so it is already clear from case law that it would be lawful for legislation to introduce a registered partnership which is, in every respect and detail, equivalent to marriage.
I understand that the issue is now not whether to allow homosexual couples something that’s equivalent to marriage, but whether to allow them into marriage itself. It could be, as Mops speculates, that the Court reiterates that this is not feasible (without constitutional amendment), holding that the constitutional understanding behind the 1949 text is that marriage presupposes a heterosexual couple. I, personally, don’t expect them to do so, however. It is already clear, as I mentioned above, that a registered partnership which is, in every respect, identical to marriage would be feasible without constitutional amendment. The dispute is, thus, a largely terminological one - whether that thing can also be called a marriage. I would think the Court would refrain from saying that something which looks like a duck, walks like a duck, and quacks like a duck must not, constitutionally, be called a duck. Especially if there is strong political support for attaching the “duck” label to it.
The same objection was raised in Canada 15 years ago by opponents of same-sex marriage. The Supreme Court rejected that argument.
Thank you. Deutsche Welle just published an article explaining some of the constitutional issues involved.