If you decide to declare a joint request, you can use the spouse split tax rate. This applies to married couples and same sex-partners. The most crucial element is that a legally binding ceremony makes this marriage/partnership.
When you utilize the spouse split tax rate, the tax office adds your income to your partners and then divides this sum by two. The percentage of income tax is then calculated based on this result. This percentage is then doubled, the result of which is the income tax you must pay as a married couple.
This is especially worthwhile if the income difference between you and your partner is unusually large.