Well, if typical white racists are correct – it is is indeed! However, I doubt if that was the primary motive for U.S. miscegenation laws, though. The main motive was fear of those blacks wanting your white daughter!
Anyway, the fact remains that blacks and maybe others had a mental age of kids in the 19th century – and in the 20th century that of teenagers. However, though, this stigma against adult-teenage sex is a new thing. In earlier times, nobody gave a shit about it.
Anyway, this stuff is a pretty complex moral issue. I mean, Thomas Jefferson scoring with someone with a 10-year old mental age – but a hot adult body! What are we to make of this?
Anyhow, the mental age wasn’t always the slave’s fault to some extent. They were forbidden to read!