Annegret Kramp-Karrenbauer of Chancellor Angela Merkel's conservative Christian Democratic Union party (CDU) told the Saarbrücker Zeitung that she was worried about the consequences of changing the definition of marriage – currently narrowly defined as being between a man and a woman in the German Constitution.
"If we open up this definition to become a long-term responsible partnership between two adults, then other demands can't be ruled out, such as a marriage between close relatives or between more than two people," she opined.
Social Democratic Party (SPD) politicians were quick to respond to Kramp-Karrenbauer, with secretary-general Yasmin Fahimi calling it "a new low for the debate on marriage for all."
"This is a blow in the face of hundreds of thousands of same-sex partnerships who stand up for one another and take on responsibilities," said Fahimi.
The SPD were joined by Free Democratic Party (FDP) secretary-general Nicola Beer, who called Kramp-Karrenbauer's words "shameless."
"She is deeply insulting homosexuals. I call on her to apologize for going off the rails."
TV comedian Jan Böhmermann took a different tack, poking fun at Saarland's countryfied reputation by suggesting the remote state would be shocked if Kramp-Karrenbauer were to ban marriage between close relatives:
Saarland geschockt: Will Annegret Katy Karrenbauer (CDU) die Heirat von „engen Verwandten“ verbieten? http://t.co/69KpRSshLZ— Jan Böhmermann (@janboehm) June 3, 2015
Kramp-Karrenbauer argued that inequalities between gay partnerships and full marriage would need to be reduced.
But she added that "at the end of this process we'll need to deal with the question of full adoption" [adoption of a child by both partners in a gay relationship, rather than just one] - something she personally is against.
A mother and a father pairing was the best way of raising a child, she said.
"We can't base this question on whether someone feels discriminated against or not – it can only be about what's best for the child."
Politicians from the SPD, Greens and Linke (Left Party) are trying to put together a majority in the Bundesrat (upper house of parliament, representing the federal states) in favour of equalizing gay partnerships with heterosexual marriage.
They hope to put Chancellor Merkel under pressure to overcome naysayers like Kramp-Karrenbauer in her own party.