Basis for penal law in Germany 
is the German “Strafgesetzbuch (StGB)“ [en: Penal Code].
This Penal Code contains two sections, which address sexual delicts:
Section 183
Acts of exhibitionism
(1) A man who vexes another person by committing an act of exhibitionism incurs a penalty of imprisonment for a term not exceeding one year or a fine.
(2) The offence is prosecuted only upon request, unless the prosecuting authority deems there to be a special public interest in prosecution which calls for ex officio intervention.
(3) The court may even suspend a sentence of imprisonment on probation if there is reason to believe that the offender will only cease to commit acts of exhibitionism after lengthy medical treatment.
(4) Subsection (3) also applies if a man or a woman is convicted of an act of exhibitionism
1. under another provision which imposes a maximum sentence of imprisonment not exceeding one year or a fine or
2. under section 174 (3) no. 1 or section 176 (4) no. 1.
Translated cit. of a statement by a German lawyer of penal law
“Definition: An exhibitionistic act requires exposing the penis to another person.
The penis does not need to be erect. If, however, only an imitation of the phallus is shown, the element of the offence is not fulfilled. The act also requires a subjective component, in which the offender is sexually excited, aggravates his sexual exitement, or gets his sexual satisfaction by the act or by observation of reaction from others. However, it is not necessary to achieve real sexual satisfaction.
Furthermore, offender and victim must be present at the same time. Consequently, it is not sufficient, when recorded images or films of the act are shown. On the other hand, acts, that are transmitted over the Internet in front of a camera are sufficient (controversial in the literature).
On the other hand, an exposure for provocation, for urination, or even as mere demonstration of a nude lifestyle, as in the case of nude joggers, is not an exhibitionistic act. Furthermore, it is generally not punishable, if the offender merely considers the possibility of being observed by another person.“
Source: “Der Begriff der exhibitionistischen Handlung“ (strafrechtsblogger.de), “Rechtsanwalt Steffen Dietrich, Anwalt für Strafrecht aus Berlin“
Judgment concerning section 183 “Exhibitionism“, Bavaria [16 June 1998]
Translated cit. of a statement in an article “Exhibitionismus“ (de.wikipedia.org)
“According to a judgment of the Bayerisches Oberlandesgericht [en: Bavarian Higher Regional Court] in 1998 (‘BayObLG (2. Strafsenat), Urteil vom 16. Juni 1998 – 2 St RR 86/98, NJW 1999, 72’), the element of an offence of exhibitionism is only fulfilled, if the disrobement serves for sexual satisfaction.“
Nudity and therefore nude sports in the public alone is therefore not punishable in Germany.
Source: “Exhibitionismus“ (de.wikipedia.org)
Also, there is an article “Exhibitionism“ written in English in en.wikipedia.org, but it is not a translation of the German one, which addresses especially the situation in Germany!
Section 183a
Causing public nuisance
Whoever performs sexual acts in public and thereby intentionally or knowingly offends common decency incurs a penalty of imprisonment for a term not exceeding one year or a fine, unless the act is liable to a penalty under section 183.
The term “public disturbance“ is thus expressly bound to a sexual act, not to mere nudity.
Note on some neighbouring countries: This restriction of the term “public disturbance’ to sexually motivated offences in the German Criminal Code does not apply in some of our neighbouring countries. In Switzerland and Luxembourg, for example, “disturbance of sleep by bellowing drunks“ is also declared as public disturbance.
