I think it is more mystical than that. "Mother" Dimble, you remember, never had any children -- yet became a mother to all her husband's students. She was a wife, and a sexual being, if she had not been a Christian, Lewis implies, she might even have been raunchy -- or at least naughty. The giantess (who is not Venus, is she?) was un-Christian/pagan, and she made a mockery of sexuality and pulled apart the special marriage bed Jane had helped prepare for Maggs and her husband. She was provacative and overtly sexual -- without any spirituality to her. If Jane, who was sensitive to the overlap of the pagan/Christian spaces, would not submit herself to the "Christianizing" authority of the Director (and thus to Christ) she would have, in place of the chaste and childless Mother Dimble, a wild and raunchy fertility goddess, a raw-edged and dangerous one at that. Does that make any sense?
Lewis is showing that what was good in pagan worship (sensuality) could be redeemed and enhanced in Christianity (the sacrament of marriage).
This is how I read it. The same with Merlin, who was sort of on the cusp between Christianity and paganism: his magic is really founded upon communication with the earth's powers, and Ransom says that even in his (Merlin's) day, that kind of magic was already passing from the world as the gods that facilitated it were growing dim and Christ was growing brighter -- but because Merlin was, himself, a Christian, he was able to revive that magic and place it in Christ's service in the moment of crisis.
I think it is a nice way to look at the old religions; that perhaps there were such creatures, and although they aren't to be worshiped, they can be redeemed and retained in Christianity as other servants of Christ.