The Federal Constitutional Court ruled on May 7, 2013 (Az .: 2 BvR 909/06 et al.) That the unequal treatment of registered civil partnerships and marriages in the case of spouse splitting is unconstitutional.
The Income Tax Act has been changed accordingly, so that registered partners can now opt for one of the tax class combinations III / V, IV / IV or IV / IV with a factor when deducting income tax. However, it is also possible to remain in tax class I.
Since the law introducing the right to marry for persons of the same sex (marriage opening law) came into force on October 1, 2017, same-sex couples can no longer enter into a civil partnership, but instead marry. Furthermore, since October 1st, 2017, it has been possible to convert a registered civil partnership into a marriage.
For further information on the possible tax classes and their combinations see links.