Art. 140 GG

Art. 140 GG reads (Highlighting in the legal text by IntactiWiki):

Basic Law for the Federal Republic of Germany
Article 140

The provisions of Articles 136, 137, 138, 139 and 141 of the German constitution of 11 August 1919 are an integral part of this Basic Law.

Art. 140 GG is a special article insofar as it includes and incorporates several articles from the Weimarer Reichsverfassung (WRV, Weimar Imperial Constitution). The following WRV articles are relevant for those affected and for intactivists:

Art. 136
(1) Civil and civil rights and obligations are neither conditioned nor restricted by the exercise of religious freedom.

(2) The enjoyment of civil and civil rights as well as admission to public office are independent of the religious creed.

(3) Nobody is obliged to reveal their religious beliefs. The authorities have the right to ask about belonging to a religious society only to the extent that rights and obligations depend on it or a statutorily required statistical survey so requires.

(4) Nobody may be forced to perform an ecclesiastical act or solemnity or to participate in religious exercises or to use a religious oath.

IntactiWiki non-binding comment:

(1) and (2) are similar sentences from Art. 4 GG and underline this again.

(3) is delicate if you associate a prepuce-amputated penis with religious affiliation to Judaism or Islam. So even if you don't want to reveal your religious belief, if you are not a Jew or Muslim, you would have to expect that e.g. in the sauna people draw conclusions about the world view.

(4) Religious circumcision advocates openly admit that circumcision in children is an act of the church, a ceremony, a "religious exercise" or even a "religious oath" — and thus clearly violates Article 140 GG in conjunction with Article 136 sentence 4 WRV .

Art. 137
(1) There is no state church.

(2) The freedom of association to religious societies is guaranteed. The association of religious societies within the Reich is not subject to any restrictions.

(3) Every religious society organizes and manages its affairs independently within the limits of the law applicable to all. It gives its offices without the participation of the state or the civil community.

[...]

IntactiWiki non-binding comment:

Relevant here is (3), which clearly states that there is no religious law outside the limits of the law applicable to everyone. Religious people can therefore not derive from Art. 4 GG the unrestricted right to do whatever they want with third parties as part of their religious practice.