There is a very real discussion of the way that we have conflated "minor" (a legal status) and "child" (a developmental state), and used that to infantilize adolescents who are very much not children...

but that discussion is not about sex, it's about the way that people abuse that legal status in order to deny adolescents normal choices that they are developed enough to make, such as what books to read, medical decisions, what they do with their property (or even the ability to own property), etc.

Stallman is using that very legitimate discussion as cover to argue about whether children (i.e. pre-adolescents) should be able to have sex with adults.

He is, at best, the worst kind of provocateur, doing this because he knows it riles people up, so that he can feign some position of superiority about not being upset about his very intellectual ^/s^ take, and at worst, desiring to enable or normalize pedophilia and hebephilia.