Again, I think it's important to remember that in 1979, the G rating did not have the "kids' movie" connotation that it has today. It was used for films that were suitable for family viewing -- movies that didn't have anything parents might not want their kids to see, like sex scenes or gore, but were not necessarily aimed at children. That's the case with TMP. It was meant to be a sophisticated science fiction film with adult appeal; it was simply kept free of content that might make adults think they had to leave the kids with a sitter when they went to see it.
Exactly. Ninety percent of movies made before 1960 or so, including most musicals, comedies, adventure films, and so on, would be considered G-rated in that they didn't contain any questionable sex or violence or adult content.
When it came to scifi, TMP would be considered family-friendly compared to, say, the likes of WESTWORLD (robot prostitutes, massacres and violence), PLANET OF THE APES (blood, violence, and nudity), LOGAN'S RUN (sex, nudity), THE OMEGA MAN, SOYLENT GREEN, ROLLERBALL, and other films of the era.
STAR WARS and CLOSE ENCOUNTERS had somewhat reversed that trend by demonstrating the box office potential of more family-friendly sf.