A straight couple in Austria who reject traditional marriage are fighting to be allowed to have a civil union.
Helga Ratzenboeck and Martin Seydl believe the law should be blind to gender and sexual orientation.
Their case is similar to that of British couple Tom Freeman and Katherine Doyle, two civil servants from London who are hoping to challenge the UK’s marriage and civil partnership laws at the European Court of Human Rights.
Austria legalised civil unions in December but they do not allow gay couples to adopt children or use IVF and artificial insemination.
One difference between civil unions and marriage is that while marriage can take six years to dissolve, it only takes three years to end a civil union.
As in the UK, only gay people may have a civil union, while only straight people can have a marriage.
Ms Ratzenboeck and Mr Seydl’s lawyer Helmut Graupner told BBC News: “The couple involved already have grown up children and are not interested in adopting.
“They are more interested in a more loose, modern form of partnership with a shorter time period for divorce and lower maintenance obligations afterwards.”
Mr Graupner added that he was also representing two gay couple who wanted the right to full marriage.
He said: “You can’t be a little bit equal, in the same way as you can’t be a little bit dead or a little bit pregnant. You can only be equal or unequal.”
Ms Doyle and Mr Freeman, the UK couple challenging the ban on straight people having civil partnerships, argue that no one should be kept out of institutions because of their sexuality.
They are looking for other straight and gay couples to join their legal challenge.