For the record, he retracted that statement:

"Many years ago I posted that I could not see anything wrong about sex between an adult and a child, if the child accepted it. Through personal conversations in recent years, I've learned to understand how sex with a child can harm per psychologically. This changed my mind about the matter: I think adults should not do that. I am grateful for the conversations that enabled me to understand why."

https://www.stallman.org/archives/2019-jul-oct.html#14_Septe...

Edit: just noticed that's from two days ago, so in reaction to this fiasco.