I've been tricked! Voyage to the Planet of Prehistoric Women is basically the same movie as Voyage to the Prehistoric Planet but with some added scenes of Venusian women swimming and sitting on rocks.
A bunch of astronauts go to Venus to look for some other astronauts. Along the way they run into lizard monsters and a plant monster, but their robot, John, helps them out when he can. Until the robot complains about having to carry their worthless hides across the lava river and then they disable his circuits. Later on, the guy that made the robot feels bad about it but not for long because this other guy won't shut up about this noise they keep hearing and he is positive that it is a beautiful Venusian woman. One of the other guys brings up the more likely scenario that it is a space monster, but nothing will kill this guy's fantasy!
These are the most carefree astronauts ever. When they are not droning on and on endlessly about imaginary space babes, they are laughing off killer plant attacks, or dressing up in alien costumes to scare each other.
Whereas Voyage to the Prehistoric Planet leaves the decision of whether or not there are girls on Venus to the viewer, Voyage to the Planet of Prehistoric Women has a bunch of hokey scenes of women that look exactly like human women and it sort of ruins the scene where one of the astronauts finds a plate with a carving of a human looking face on it, which is probably one of the best parts of Voyage to the Prehistoric Planet. They also added a completely unnecessary but constant voice-over, which is almost like listening to a really boring commentary track. The guy will describe what he is doing on the screen in the most boring way possible:
Guy is swimming
"While swimming I encounted something."
Sea monster swims out from behing rock
"But it was just a harmless octopus."
Sea monster looks dejected and swims back behind the rock
The only purpose this movie serves, other than providing the hilarious voice overs, is to make you realize that Voyage to the Prehistoric Planet in hindsight is not so bad. And that is my review, of both movies.