I don't know if he was forced but according to an interview he gave, he had to be persuaded:
Matschenz: "Als mir die Rolle angeboten wurde, habe ich mich mit Christian getroffen und war extrem unsicher. Ich bin nicht unbedingt ein großer Freund von Nacktszenen. Ich hatte da eine furchtbar arrogante Haltung und habe vieles kritisiert.
In Englisch: "When I was offered the role, I met with Christian [Petzold, the director] and was very insecure. I am not a big fan of nude scenes. I had a very arrogant attitude and critizised a lot."
He then tells that the director sent him some of his films to show him how he handles nudity. In the end, they came to an agreement.
You can finde the whole Interview in German here:
Interview with the Weser Kurier [German]