It is just the addition of one short phrase, but it is set to mark a revolutionary change for same-sex couples in Germany. According to the marriage clause of the Civil Code, "marriage is entered into for life." After the Bundestag - Germany's lower house of parliament - passed the vote on Friday, the clause is soon to read "Marriage is entered into for life by two persons of different sexes or the same sex."

But is this addition really sufficient? Should not Germany's Basic Law - the constitution - be changed? Here are a few of the most significant questions on the issue.

Who is against marriage equality?

Politicians in particular from Angela Merkel's conservative Christian Democratic Union (CDU) and its Bavarian sister party, the Christian Social Union (CSU), do not believe that changing the Civil Code is sufficient to make same-sex marriage legal. From their point of view, it would be necessary to change Article 6 of the Basic Law, which reads: "Marriage and the family shall enjoy the special protection of the state."

A total of 225 Bundestag lawmakers from the CDU and CSU voted against allowing same-sex marriage, including Merkel herself and some of the party's heavyweights. Interior Minister Thomas de Maiziere, for example, sees the issue as one for the country's highest court. "I voted against this law. One reason for this is that, in my opinion as a lawyer, we needed a constitutional change," the CDU politician told the Bild am Sonntag tabloid. In addition, for him, marriage is also a "connection between man and woman."

Finance Minister Wolfgang Schäuble, also of the CDU, sees the issue similarly. In light of the existing case law of the Federal Constitutional Court, marriage is a combination of husband and wife, he told Deutschlandfunk radio.

What is the current case law of the Federal Constitutional Court?

Advocates of marriage equality point out that the Basic Law does not explicitly speak of marriage as an "alliance of man and woman." Opponents, in turn, refer to the interpretation by the Federal Constitutional Court. In its previous decisions, the Karlsruhe-based court made clear that the term "marriage" in Article 6 is intended to mean marriage between husband and wife.

Whether the Constitutional Court kills marriage for all, however, is questionable. Its judges have repeatedly emphasized in their previous decisions that social change is taken into account in the interpretation of the Basic Law.

What do lawyers say?

Among lawyers, the possible outcome of such a constitutional case is controversial. The former president of the Federal Constitutional Court, Hans-Jürgen Papier, considers opening up the institution of marriage to same-sex couples as unconstitutional: "If you want to open marriage, you have to change the Basic Law."

The Karlsruhe judges have emphasized in previous decisions that a marriage, in the sense of the Basic Law, is only the "union of a man with a woman into a permanently established cohabitation." According to Papier, current social climate or social acceptance does not change this constitutional understanding.

Other lawyers see this differently. Joachim Wieland, Rector of the German University of Administrative Sciences in Speyer, is of the opinion that opening marriage up to same-sex couples by a simple law amendment does not violate the constitution, because the latter does not define the conceptual framework of marriage.

Christoph Degenhart, a politician from Leipzig, doubts that Karlsruhe has "enough stamina to resist the social trend." He thinks it conceivable that the Constitutional Court would seek a "pragmatic solution." The court could argue that no one may be deprived of the right to marry.

Who is allowed to sue?

Because same-sex marriage does not concretely disadvantage anyone, private individuals are not allowed to contest it in the Constitutional Court. Only the federal government, a state government, or a parliamentary request supported by quarter of the Bundestag can apply for a review of the law in Karlsruhe. For a constitutional action, 158 Bundestag lawmakers would therefore have to sue. This would be conceivable, since 225 parliamentarians voted against the law. It would also be conceivable that one of the seven state governments led by the CDU or CSU could appeal to Karlsruhe.

The right-wing populist Alternative for Germany (AfD) party has vowed to issue a complaint at the Constitutional Court - but the attempt is in vain. The AfD is not represented in the Bundestag, nor is it part of a state government.

Is this a constitutional act?

Whether the Karlsruhe judges will really end up deciding on same-sex marriage is questionable. Yet many political heavyweights in Merkel's party support such a process.

On the other hand, a clear majority of the German population is in favor of allowing same-sex marriage. Each Bundestag lawmaker, therefore, will have to consider whether he or she chooses to chart a rather unpopular course to Karlsruhe, given that Bundestag elections are approaching this fall. The chancellor herself will also have little interest in it. It is most likely that one of the German states led by Merkel's party will take up the issue in the Constitutional Court.