It's largely due to Paragraph 175 of the German Penal Code

Sorry for Wikipedia links, but several of the sources are available for reference.

The law was on the books in the late nineteenth century, but was expanded once the Nazis took control.

I'm including the excerpt listed in the wiki article below, though the specific source is no longer available:

CW, obviously, but putting a heads up, just in case.

While the Nazi persecution of homosexuals is reasonably well known today, far less attention has been given to the continuation of this persecution in post-war Germany. In 1945, after the concentration camps were liberated, some homosexual prisoners were recalled to custody to serve out their two-year sentence under Paragraph 175. In 1950, East Germany abolished Nazi amendments to Paragraph 175, whereas West Germany kept them and even had them confirmed by its Constitutional Court. About 100,000 men were implicated in legal proceedings from 1945 to 1969, and about 50,000 were convicted.

Some individuals accused under Paragraph 175 committed suicide. In 1969, the government eased Paragraph 175 by providing for an age of consent of 21. The age of consent was lowered to 18 in 1973, and finally, in 1994, the paragraph was repealed and the age of consent lowered to 14, the same that is in force for heterosexual acts. East Germany had already reformed its more lenient version of the paragraph in 1968, and repealed it in 1988.

This article is from auschwitz.org and goes into more detail about the experience.

At the end of the war, the majority of homosexuals were freed from camps in both parts of divided Germany. However, the homophobia directed against them by the public remained strong. Article 175—the basis for sending thousands of innocent people to concentration camps—remained in force in the DDR until 1967, and in West Germany until 1969. There were some American and British lawyers who demanded that homosexuals convicted under Article 175 serve out their full sentences. For instance, if someone had been sentenced to eight years and served five years of the sentence in prison followed by three years in a concentration camp, the lawyers demanded that the person return to prison to serve out three years. The number of people forced to “complete” their sentences in this way is not known. To this day, no financial compensation has been paid to the victims of Nazi homosexual policies, despite the fact that the German government offered compensation to victims of Jewish ethnicity, political prisoners, and other groups that survived the concentration camps. Only the homosexuals were passed over. Many people deny that the homosexuals have a right to any such compensation, stating that victims with an alternative sexual orientation were justly imprisoned, and “had no one but themselves to blame.”

There is a lions led by donkeys episode about it that’s pretty good