If you get married, you will automatically be placed in tax class IV at the time of the marriage if

• you are not permanently separated and
• your place of residence or habitual abode is in Germany.

The registry office will automatically inform the Federal Central Tax Office about the marriage.
Alternatively, you can apply for the tax class combination III/V. You can also choose the tax class combination IV/IV with factor.

Do you not want your employer or your wife's or husband's employer to be informed of the change in marital status? Then you or your wife or husband can apply to your local tax office for tax class I to be taken into account. The effect with regard to wage tax deduction then corresponds to tax class IV. Alternatively, you can block your employer from accessing the electronic wage tax deduction features.
If you do this, however, your employer is obliged to tax your wages according to the least favorable tax class VI.

Background information on tax class selection by married couples:
As a rule, employers only know the wages of the employee working for them, but not those of the wife or husband. Consequently, only an employee's wages can be used as a basis for deducting income tax. The wages of both spouses can only be combined after the end of the year as part of the income tax assessment. Only then will the correct annual tax be calculated. It is therefore often unavoidable that too much or too little income tax is withheld in the course of the calendar year. In order to come as close as possible to the annual result, married couples can choose between 2 tax class combinations (IV/IV as a statutory rule and III/V on application) and the factor method.
Which tax bracket combination is the best? There is no general answer to this question, but it depends on your needs and the circumstances of the individual case.

Combinations IV/IV or III/V:
The tax class combination IV/IV assumes that the couple earn approximately the same amount. The tax class combination III/V is designed so that the sum of the tax deduction amounts for both spouses roughly corresponds to the joint annual tax if the wife or husband with tax class III earns 60 percent and the wife or husband with tax class V earns 40 percent of the sum of both spouses' earned income. As a result, the tax deduction for tax class V is proportionally higher than for tax classes III and IV. This is also due to the fact that in tax class V the basic allowance for the minimum subsistence level is not taken into account, but is doubled in tax class III. If the ratio of actual wages does not correspond to the statutory assumption of 60:40, this can result in additional tax payments. For this reason, there is an obligation to submit an income tax return when choosing the tax class combination III/V.

Combination IV/IV with factor:
Instead of the tax class combination III/V, you can choose the factor method in addition to the tax class combination IV/IV. The tax class combination IV/IV in conjunction with the factor to be calculated by the tax office ensures that the basic tax-free allowance applicable to each spouse is taken into account when deducting income tax by applying tax class IV and that the income tax to be withheld is reduced by applying the factor of 0,... (always less than 1) in accordance with the effect of the splitting procedure. The factor is a tax-reducing multiplier that is calculated from the effect of the splitting procedure if the spouses have different wages. Allowances are included in the factor. The factor is automatically provided to the employer as an electronic wage tax deduction feature.

Note:
Since the law introducing the right to marriage for persons of the same sex came into force on October 1, 2017, no new civil partnerships can be established in Germany. Same-sex couples have been able to marry each other since this date and are therefore on an equal footing with opposite-sex couples.
Existing civil partnerships can be converted into a marriage. However, there is no obligation to do so. Existing civil partnerships can be continued in their current form.