In the current version of the game the 'deviant' aspect of lewdity puts the emphasis on maritime fauna, I'd say. In the beginning, it will be difficult to trigger events that will initiate progress in that stat, but my suggestion is to improve your swimming and get into the water (ocean, forest lake) often. Your first deviant encounters will most likely be playful dolphins and getting rubbed by something vaguely phallic while swimming. Once you reach level 1, another corrupting event emerges, when you're awake in your bedroom between 3-4 am and an eerie light emerges from your mirror. You can first watch mysterious mirror tentacles sprouting from the mirror (+ Dev lv 1) and once you've reached level 2, you may lick them, which will further increase your deviancy and also act as an aphrodisiac, thus making night mischief all the more interesting.
Further progress will be helped greatly with Skulduggery skill high enough that you may break into houses at night. Quite often these will be guarded by dogs and you'll be able to 'calm them down' with deviancy lv. 3 and above. From thereon, Deviancy works the same as Promiscuity, meaning the higher level, the more daring (consensual) acts you may perform (jerking off, kissing, licking, sucking, fucking), with only the most extreme of these furthering your progress in this regard. Remember that only the deviant acts committed of your MC's own volition will increase this proclivity, so no matter how many times they're raped and otherwise molested by Mother Nature and Her children, they will not going to acquire a taste for it.
Tentacle encounters will also get more interesting once you've reached lv 3, so good swimming practice will come in handy. Lv 4 of Deviancy will allow you to offer yourself to the Krakken in the Lake Ruins (and the Dendron in Deep Forest), and once you're past lv 5, you can practically sexually harass it, which at this level of perversity (other lewd stats also high enough) will become a stress relieving activity and if you 'convince' either a large or thick tentacle to bugger your arse to completion, there is likelyhood you will become anally pregnant.