Both Uses of

perversion
in
The Screwtape Letters

And it is also to be noted that the Trough sexuality is subtly different in quality from that of the Peak—much less likely to lead to the milk and water phenomenon which the humans call "being in love", much more easily drawn into perversions, much less contaminated by those generous and imaginative and even spiritual concomitants which often render human sexuality so disappointing.†
Chpt 9 *

perversions = things that have been converted so they are not what they should be

Now this idea must be used by us to encourage once again the conception of a "historical Jesus" to be found by clearing away later "accretions and perversions" and then to be contrasted with the whole Christian tradition.†
Chpt 23

Definition:

the conversion of something so it is not what it should be -- especially a sexual practice considered unacceptable by society