Does cybersex count as pornography?
The law does not define pornography, so this is up to an interpretation of the courts. However, it appears to be common consent that a depersonalisation of the subjects is an essential aspect. For example, this verdict notes (translations and emphasis mine):
Pornographie liegt nach der Rechtsprechung des Bundesgerichtshofs und der Oberlandesgerichte vor, wenn eine Darstellung unter Ausklammerung aller sonstigen menschlichen Bezüge sexuelle Vorgänge […] in den Vordergrund rückt […] (vgl. BGHSt 37, 55 (60); 32, 40 (44 ff); OLG Karlsruhe NJW 1974, 2015 (2016); OLG Düsseldorf NJW 1974, 1474 (1475); ebenso BVerwG NJW 2002, 2966 (2969)).
[…]
Hinzukommen muss deshalb als weiteres Kriterium die sog. „Apersonalität des Geschlechtspartners“ (vgl. Erdemir, MMR 2003, 628 (631)). Die Darstellung muss mit anderen Worten durch eine Verabsolutierung sexuellen Lustgewinns unter gleichzeitiger Entmenschlichung der Sexualität geprägt sein. Pornographie ist danach anzunehmen, wenn der Mensch im Rahmen der Darstellung zum bloßen, auswechselbaren Objekt sexueller Begierde degradiert wird (vgl. S/S/Lenckner/Perron/Eisele, StGB 27. Aufl., § 184 Rdn. 4; MK-Hörnle a.a.O., § 184 Rdn. 15; jeweils m. w. Nachw.). […]
According to the Federal Court’s and the state courts’ jurisdiction, a depiction is pornography if it emphasises sexual procedures disregarding all human aspects [references]
[…]
Therefore, a necessary further criterion is the so-called “depersonalisation of the sexual partner” [reference].
With other words, the depiction must be dominated by a absolutisation of sexual pleasure and a simultaneous dehumanisation of sexuality.
Therefore pornography has to be asserted if a depiction reduces human beings to mere replaceable objects of sexual desire [references].
(Also see the German Wikipedia for another summary and further references on the subject.)
Now, cybersex usually, if not inherently, is a personal procedure, as opposed to a depersonalised one. Therefore I would not consider it to fall under the quoted definition of pornography.
What other laws do regulate cybersex?
The law governing the age of consent is § 176 StGB (translation), which includes:
(4) Mit Freiheitsstrafe […] wird bestraft, wer […] auf ein Kind mittels Schriften (§ 11 Absatz 3) oder mittels Informations- oder Kommunikationstechnologie einwirkt, um […] das Kind zu sexuellen Handlungen zu bringen, die es […] vor dem Täter […] vornehmen […] soll, […]
(4) Whosoever […] influences a child with written materials (section 11(3)) or with information or communication technology to induce him to engage in sexual activity […] in the presence of the offender […] shall be liable to imprisonment […].
*Translation adapted from this one to reflect recent changes to the law.
Note that Kind (child) was established as Person unter vierzehn Jahren (a person under fourteen years of age) before.
I would interpret this to capture cybersex. The only thing about this that may be debatable is whether the sexual activity during cybersex happens in the presence of the offender (i.e., the other person).