Mostly because the idea that children are sexual and can consent to sex was carried by movements and organizations that later became the German Green party. They're socially progressive (and economically center-left, I'd say, but clearly much less on the left spectrum about economic issues) and are generally considered to be part of "the left".
Helmut Kentler, the psychologist mentioned in the article, was also generally considered to be part of "the left".
Helmut Kentler, the psychologist mentioned in the article, was also generally considered to be part of "the left".