The transporter accident isn't horrific as portrayed. It's funhouse mirror stuff with sparkles and an admittedly scary sound.
Sure, Ilia claims she has an oath of celibacy but there's no sign her Deltan sexuality-know to-us-only-by materials-outside the film has any affect on anyone in the theatrical cut. Hell, you can't see her in the shower as anything but a blurry silhouette.
For 1979 that has G written all over it. The Black Hole, with its ominous Maximillian and its spinning blades, Durant's murder by same (even if we don't see it), and images of the zombified Cygnus crew are more obviously reasons for the PG rating it got. It's more viscerally frightening than TMP, which expresses almost all of its violence via lightning bolts and sparkles.