From the time he came forward publicly until the end of his life, Seel was active as an advocate for the recognition of homosexual victims of the Nazis—and notably of the forgotten homosexual victims from the French territories of Alsace and Moselle, which had been annexed by Nazi Germany. Seel came to be known as the most outspoken activist among the men who had survived internment as homosexuals during the Third Reich. He was an active supporter of the Mémorial de la Déportation Homosexuelle, a French national association founded in 1989 to honor the memory of homosexuals persecuted by the Nazi regime and to advocate formal recognition of these victims in the ceremonies held annually to commemorate citizens and residents of France deported to the concentration camps.