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

• you do not live permanently apart and
• Your domicile or habitual residence is in Germany.

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

Your employer or the employer of your wife or husband should not be informed about the changed marital status? Then you or your wife or husband can submit an application to your local tax office for consideration of tax class I. The effect with regard to the deduction of income tax then corresponds to tax class IV. Alternatively, you can block the employer from retrieving the electronic wage tax deduction characteristics.
If you do this, however, your employer is obliged to pay tax on your salary according to the most unfavourable tax class VI.

Background information on the choice of tax class by married couples:
As a rule, the employer only knows the salary of the employee working for him, but not that of the wife or husband. Consequently, an employee's income tax deduction can only be based on his wages. The wages of both spouses can only be merged after the end of the year within the framework of the income tax assessment. Only then does the applicable annual tax arise. 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, the spouses have 2 tax class combinations (IV/IV as a statutory rule and III/V on request) and the factor method as options.
Which tax bracket combination is the best? This cannot be answered in general, but depends on your needs and the circumstances of the individual case.

Combinations IV/IV or III/V:
The tax bracket combination IV/IV assumes that the spouses earn approximately the same amount. The tax class combination III/V is designed in such a way that the sum of the tax deductions 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 40 percent of the total earned income of both spouses. 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 tax-free allowance due for the subsistence minimum is not taken into account, but is taken into account in tax class III in double amount. If the ratio of actual wages does not correspond to the legal assumption of 60:40, tax arrears may be made. For this reason, when choosing the tax class combination III/V, there is an obligation to submit an income tax return.

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 for each wife and each husband the basic tax-free allowance applicable to him or her is taken into account in the wage tax deduction by applying tax class IV and that the wage tax to be withheld is determined by applying the factor of 0,... (always less than 1) reduced according to the effect of the splitting process. The factor is a tax-reducing multiplier, which is calculated from the effect of the splitting procedure in the event of different wages of the spouses. Allowances are included in the factor. The factor is automatically provided to the employer as an electronic wage tax deduction feature.

Hint:
Since the law introducing the right to marry for persons of the same sex came into force on 1.10.2017, no new civil partnerships can be established in Germany. Since then, same-sex couples have been able to marry each other and are thus on an equal footing with opposite-sex couples.
Existing civil partnerships can be converted into marriage. However, there is no obligation to do so. Existing civil partnerships can be continued in their current form.